Using a device with no vendor-provided firmware security patches doesn't seem like a good idea to me. Also, the Pixel 8a gets 7 years of updates. That's plenty of time, and most people get a new phone after a much shorter time.
I haven't tried OxygenOS, but I used Oppo's ColorOS (which is basically the same thing; there are only a few minor differences, since OnePlus is a subsidiary of Oppo) and it was terrible. The entire OS felt like a buggy mess, and the battery life was terrible because of all the garbage running in the background. I spent an entire weekend removing all the bloatware, but the experience still sucked. That was 2 years ago, after my iPhone XS broke, and I just needed a new phone quickly. My carrier had a deal at the time, where I got the Oppo phone almost for free (very small direct payment, no monthly payments), so I took it. After about a month, I just couldn't stand it anymore and returned the phone. Then I bought a Google Pixel 6 Pro, flashed GrapheneOS on it, and it's been great ever since. I'm never using a Chinese phone again. A friend showed me his Xiaomi phone and said that it's basically unusable with the stock ROM. Phone manufacturers should really stop messing with the goddamn OS, they always manage to turn it into a giant pile of shit. I'll definitely stick with (preferably degoogled) operating systems that are as close to AOSP as possible, like GrapheneOS.
Both are great. Unraid makes things really easy with their Community Apps feature. On the technical side, I prefer TrueNAS Scale because it's based on Debian, whereas Unraid is based on Slackware Linux. TrueNAS Scale is fully FOSS, whereas big parts of Unraid are proprietary. But there are more guides and tutorials for Unraid, as it seems to be the more popular option. If you're going to install Unraid, definitely check out Spaceinvader One on YouTube, he's got some awesome videos on the topic.
I know, I mean I haven't really played it much and haven't progressed far. I feel like I missed out on a huge part of the game. Will definitely get back to it some day.
Unraid would be my first suggestion as well. But if you prefer something FOSS, check out TrueNAS Scale. (It is important that you go with TrueNAS Scale, not Core. TrueNAS Core is the continuation of the former FreeNAS, which is based on FreeBSD. Since it's not a Linux system, it doesn't support Docker. TrueNAS Scale is based on Debian Linux and much closer to Unraid, it has full support for KVM Virtualization and Docker containers.)
Honestly, no. Whenever I see late notifications its usually on a degoogled phone
Usually, yes. But there is one exception: GrapheneOS. Since it allows you to use Sandboxed Google Play services, notifications work just like on any other Android device, you just need to disable battery optimization for Play services. But unfortunately it is only compatible with Google Pixel devices.
Using a device with no vendor-provided firmware security patches doesn't seem like a good idea to me. Also, the Pixel 8a gets 7 years of updates. That's plenty of time, and most people get a new phone after a much shorter time.