I honestly would want the upgrades to be less impactful. or have a curve where early upgrades give more of an edge than like lvl 30. in that way newcomers can get into it easily but people who play 4 hours a day wont roflstomp purely because they have more points.
Or balance the zombie/player count also based on the levels. so make sure that the average level is same on survivor and zombie side. because now if like 3 high level players join zombies at the start. the game has basically ended. especially on objective maps
Zombie teams should actually already choose higher ranked player(s) together with lower ranked so it doesn't end up pros vs noobs and from what I have seen it does work out pretty much (zombies win most of the time too). Regarding upgrades, higher ranked players already get cut a lot of their power by prestiging. Mike for example has passed 200 million points and still isn't able to max all upgrades. Keep in mind that this mod is over a year old so it's hard to find balance between helping newbies and making your playtime for existing playerbase meaningful. You don't wanna grind millions to tens of millions points without it paying off somehow. Good thing more perks are implanted over time with unique upgrades unrelated gimmicks.