You got some pretty decent responses already but hopefully I add some useful information. You don’t have to study computer science theory at all to be private. You don’t even have to learn how to code, let alone to develop software. And I say this as someone who learned how to code in high school and took some CS courses in college as well as the required calculus courses. It is simply overkill and frankly you are doing yourself a disservice if you are focusing on that stuff if your sole priority is to be private digitally. Maybe it could eventually be useful to learn some bash or Python/Ruby/Lua/JS scripting but you sure as hell do not have to start with learning C.

If you just want to have some privacy in general you can start with dealing with some of the worst offenders, but really the best thing to do is to threat model first. What that means is identifying what/who you are concerned about. Who is really your opposition? Is it a specific Big Tech company (maybe your “evil ex” works there), all/most of (American) Big Tech, a certain government (agency) (which government/country), some unhinged and tech savvy stalker, organized crime or an oligarch? Or some combination of the aforementioned? You really need to figure this out first to be effective. Don’t get caught in the trap of trying to defend your privacy against everyone. And remember that privacy and (cyber)security are not the same thing. A Pixel with stock ROM or Chromebook is very secure but not private at all from Google. Conversely, you can have some 10 year old computer with original UEFI firmware running a Linux distribution that hasn’t received security updates in 5 years with the best configuration and practices for privacy from Google. That would be very private from Google but absolutely not secure.

Also avoid the trap of simply adopting what someone else says is “the best”. What is “the best” (assuming that that is even objectively true in that particular case) for one person may not be a good option for someone else at all. What is the best option depends on your threat model and what compromises you need/want to make.

I would recommend checking out Rob Braxman’s channel (on YouTube, or preferably Rumble or Odysee). Not saying he is the end all be all but at the very least he can teach you about the importance of browser isolation. If you want to defend against (American) Big Tech (and/or their partners) you need to understand how their tracking works.

Regarding the Optiplex, I would check out the 3050 micro (it can be Libreboot’ed); they are pretty cheap on eBay. You don’t necessarily have to buy a ThinkPad, there are a lot of options with good Linux compatibility (many business computers optionally came with Linux preinstalled even, not just ThinkPads or Lenovos). Just do not buy an HP (I used to like and own them) as you cannot permanently disable Absolute Persistence on them. Don’t know about more niche brands but you can at least generally with Lenovo and Dell.

Naomi Brockwell https://www.nbtv.media/episodes does a good job of blending accessible presentation of privacy issues with technically viable solutions. Recently she’s been more on an advocacy tack, but there are some gems in the back catalogue that explain not just why you should care but the sorts of products/software you can use to address your concerns. She provides suggestions, but you should take those as just an idea and develop your own answers!

So, in between watching those videos find out all you can about how Debian (for servers) and OpenWRT (for routers) work from their websites, and use $preferred_search_engine to learn about why Proxmox, Unbound, Postfix, Dovecot, XMMS, WireGuard, Nextcloud can help improve your privacy.

Get that used Optiplex, install a Linux on it and begin experimenting! Don’t worry about the perfect hardware config yet. You can source other parts if you feel you really need them later. Although more RAM is always good, but you knew that already.

Along the way you will want to learn enough to decide whether you prefer VMs or Containers, or a blend; which filesystem(s) you prefer; which distributions you are going to deploy; which backup system you are going to implement; whether Apache or Nginx; and whether you like systemd or want to simply blast off and nuke it from orbit. You may also want to take a look at Ansible now. And Git. Git has saved my server config bacon more times than I care to remember.

Not sure quite where to recommend you look for bang up to date introduction to Linux networking stuff these days. The Linux Networking Howto was hopelessly out of date ten years ago. The distribution howtos are pretty good on at least the basics. Wikipedia might be useful on more advanced topics. And then searching for specific problems or needs normally turns up some learned responses on stackexchange or equivalent.

And as one 90s kid to another, if you haven’t read Permanent Record by Edward Snowden yet, consider getting a copy. He grew up at the same time and does a great job of explaining why this journey matters.

I get very few privacy vibes from this post. It seems like you are more interested in tinkering. Which is fine, but has very little to do with privacy directly.

Examples, you don’t need to know how the IP stack works in order to have privacy. You know that a VPN is just a shifting of trust and something like the tor network would probably be better.

You mentioned flashing routers and DDWRT is open source router firmware and you should be able to add DNS blocks like a pi-hole or equivalent block list into that kind of software in order to stop ads and online tracking.

This is the resource I go to when I need to check something, and it should be excellent for learning more on some privacy topics.

https://www.privacyguides.org/en/

After that you might need to go find another resource for the specific technical topics. They have a discussion section, so you can ask for recommendations there?

Pixel with GrapheneOS will have bigger impact for privacy… Buy used, refurb, etc

Mental Outlaw explains privacy in a simple way, there are legit service that have proved their good and bad relationship with their users.

Like proton and telegram will give out your IP address

Others may not

Just keep in mind it’s a path, take your time.
Also, what may be important to you may not be for someone else and vice-versa, so look up and read up on threat modeling. This should probably be first on your list.

mini pc before the isp modem to filter dns

You mean after the modem and before your network? Then that solution is feasible, otherwise it really depends how The Net enters your domicile, odds are coax or fibre…
You may be able to flash your modem+router+switch black box the ISP provides, but YMMV on that one.
Regardless, you can set-up that mini pc inside your network as the authoritative DNS server to act as a DNS sinkhole — i.e. using a PiHole.

I have a pc w dual boot

Suggestion: linux-only with windows inside a virtual machine; other virtual machines can be used for self-hosting also.

backup data

Use 3-2-1 and make sure your backups are working.
Look into RAID and LVM or, better yet, ZFS.

dell optiplex

Lenovo Thinkpads tend to have a following in linuxland, but that’s personal preference.

What am I missing? Where should I look?

Techlore has a series of videos on the topic and, along with the new oil, they have a weekly video on security and privacy. Non-tech-friendly. There are quite a few podcasts on these topics.

You can further your privacy by using frontends to common abusive sites (which i link to, sorry), such as invideous for youtube, etc.

A programming language is always interesting, linux is mostly based on C and it’s not a bad start. I’ll duck for cover now. Shell scripting — bash in particular — will get you a long way, as well a command-line text editor to change config files.