BitWarden+PiHole+NextCloud+Wireguard combined will add to like maybe 100MB of RAM or so.

Where it gets tricky, especially with something like NextCloud, is the performance you see from NextCloud will depend tremendously on what kind of hard drives you have and how much of it can be cached by the OS. If you have 4GB of RAM, then like 3.5GB-ish of that can be used as cache for NextCloud (and whatever else you have that uses considerable storage). If you have tiny NextCloud storage (like 3.5GB or less), then your OS can keep the entire storage in cache, and you'll see lightning-fast performance. If you have larger storage (and are actually accessing a lot of different files), then NextCloud will actually have to touch disk, and if you're using a mechanical (spinning rust) hard drive, you will definitely see the 1-second lag here and there for when that happens.

And then if you have something like Immich on top of that....

And then if you have transmission on top of that....

Anything that is using considerable filesystem space will be fighting over your OS's filesystem cache. So it's impossible to say how much RAM would be enough. 512MB could be more than enough. 1TB could be not enough. It depends on how you're using it and how tolerant you are of cache misses.

Mostly you won't have to think about CPU. Most things (like NextCloud) would be using like <0.1% CPU. But there are some exceptions.

Notably, Wireguard (or anything that requires encryption, like an HTTPS server) will have CPU usage that depends on your throughput. Wireguard, in particular, has historically been a heavy CPU user once you get up to like 1Gbit/s. I don't have any recent benchmarks, but if you're expecting to use Wireguard beyond 1Gbit/s, you may need to look at your CPU.