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.
Don’t buy a TV anymore. Seriously though with the direction things have been going in the “tech world” for the last couple years (maybe even decade) it is probably better to start adjusting to some level of digital minimalism. For some of us it will become a necessity for financial reasons anyway…