I'm in the same boat, as I am pretty new and just starting to build up my gear. About a year ago I decided to attempt a Proton Pack scratch build, but decision paralysis, having a difficult time figuring out where to start, and my general lack of experience kind of led me to keep putting it aside. Proton pack projects are a pretty difficult place to begin at.
In August, Spirit Halloween released a ghost trap, and when I saw it I decided I needed to mod it.
Progress has been pretty good, mostly because it was a relatively simple (keyword here being relatively, because it's still pretty invovled!) but more importantly very paced project where I had the chance to more or less decide along the way what I wanted to change and what I wanted to keep the same. What's more, I have used a ton of resources from
countspatula's 3d printed trap, so having documented details on building a similar prop has been incredibly helpful.
I know you're talking about Proton Packs, but I would recommend starting as simple as possible. Whether that be with the spirit halloween proton pack and some mod kits to keep it simple and straight forward, or with a really nice pack shell to build off of (though that is going to be more expensive and pretty involved). Either way, starting off with a proton pack scratch build just kind of left me feeling like a failure, but moving to something more straightforward and introductory helped me get my feet wet, and I'm really feeling positive about building out all my stuff now.
Also, if you get the SH Proton Pack and just mod a few things on it, it can be an ongoing project that immediately provides value. You could buy it and wear it day 1 and it will wow most people. So the instant gratification is pretty helpful. What's more, that proton pack doesn't become worthless if you later build a full-sized scratch build. There are plenty of situations where the smaller size of the SH proton pack is desireable over a bigger, heavier full-sized pack.