I'm reading through the 4e book again to try to convince myself "maybe it's not as difficult as I remember."
Nope. It's bad.
I think you’ve got a few of these ideas a bit screwy. They aren’t quite as you describe them, though I can see how a casual reading could lead to them.
Here are things that don't work for me in the system.
1) Fate/Fortune; Resolve/Resilience
Having a pair of similar sounding names for characteristics that do very similar things. It's bloated and unnecessary.
You really only use Fortune (rerolling tests) and Resolve (ending conditions). One being luck the other bad-assness. Fate and Resilience are used a couple of times in a campaign to stop you dying or choose the outcome of a die roll. But spending them permanently reduces your fortune and resolve. Believe me, your player will be able to keep track of Fate points when they literally save their life.
2) Success Levels
Roll then subtract your roll from your ability score, round up, get a number and compare to a chart to see how many successes you've earned. There must have been a better way to do this.
As has been said. You subtract the tens die from you stat’s tens figure. You literally compare two single digit number to work our Success Level. To my knowledge there are no charts to reference, except for unusual skills like Leadership say.
3) Advancements, Spending XP
You can increase these certain things, but only at certain points, and only to a certain degree, then you need to "complete" that class's path, pay more XP to start another class. WTH?
The XP system is very elegant, constant development in an extremely granular organic way. The career system works to control this by specifying key skills and stats you can improve. You don’t have to complete a career - it just uses less XP if you do one career at a time (it doesn’t save that much) you can also spend XP outside the career of your GM agrees it makes sense. There’s no limit to how far you can develop your careers skills and allowed stats. You could spend your entire campaign as a wizards apprentice if that’s what you wanted.
4) Advantage/Disadvantage, Easy/Challenging/Hard rolls
You can roll two percentage dice and take the better result if you have advantage (or worse if you have disadvantage). Or you can get a bonus/penalty to the roll depending on circumstances? Why in the name of hell do you have two mechanics to do the same thing?
Not sure where you’re getting the reroll s from. That’s not how advantage works in 4e. It’s an accruing bonus you get in combat to represent the momentum of the fight and to speed things up.
It sounds like you’re conflating this with test difficulty which is generally +0 in combat or +20 outside the pressure of combat unless there are particularly difficult circumstances.
5) Bonuses after Successful Checks
You have to keep track of all your successes in a row to get bonuses to future die rolls. There's plenty to keep up with in a combat. As a GM, am I supposed to also track this for all enemies? Bad mechanic.
This is only for advantage - which players would normally track themselves. Also dramatic tests where success could take several rounds, like picking a lock. Most tests I see are simple or opposed just like in 5e.
I highly recommend the Ratcatchers Guild on Discord. You can find the link on their Facebook page. They cover all editions of WFRP and generally answer questions within a few minutes of posting. You can also search for pretty much any rules query you have. It will be in there!
Either way good luck. I’ve ran Shadows over Bogenhafen in Pathfinder and 5e. Just cut out magic from PCs and it’s not much different to running in Middle Earth. If you want a simple version of 5e then use Cubicle 7’s AIME. It will get you through.