When I think about player power scaling in terms of this game, I don't think about a regular wastelander shit kicking radroaches. Im thinking about the players being a team of Fallout player characters. Damn near superhuman from the get-go (the courier's invincible brain matter, the vault dwellers absurd level of bravery, the chosen one's divinity) and incredibly strange in most company. These characters, thanks to tricks, all have a "build" in a very 5e feeling sort of way. They all have a quirk that they do very very well, but generally suck at other things until way later.
I would expect from a design standpoint for you to lean into this. The gunslinger should be hitting pretty consistently from the start, the hacker being able to do some pretty crazy stuff from the get-go, etc. I would think a lot of the limitation on the characters would be based on gear. IE the gunslinger can't kill a Deathclaw with a .357 magnum, the hacker can't pilot a robot without a set of control modules, etc. A solution similar to Mausritter, where you need to spend a lot of money or have certain loot to take on a certain subset of enemies would feel very fallout in that you spend a lot of the game in pursuit of gear to take on the big bad.
So TL;DR I would push some degree of character progression onto neat gear, so they gradually gain small amounts of power through XP, but move up in total potential via cool gear. Initial thoughts:
- Damage dice threshold for enemies (defeating the enemy in alternative ways still viable) make this number known to players
- Items that give stat increases either permanent or contextual
- Drugs that give characters the ability to defeat stronger enemies, sooner, at a consequence.