Xcom does a flat percentage roll every time a unit sees another unit do an action, based on your current TUs, enemy TUs, and the reaction stats of each party. I like the uncertainty of this but It can be a bit silly or unrealistic at times, it also encouraged moving very slowly and keeping a large bank of TUs at the end of your turn to maximise reaction fire, at higher level play this gets tedious as optimal play is to inch forward and punish enemies who stray into your field of view, it also made reactions one of the most powerful stats since it protected you on both turns. I like the mechanic a lot and it functions well to make both turns exciting, but I didn't like the choice structure it presented (take huge risks and face dire consequences if you want to finish missions in reasonable amounts of time, or play boring and slow). I also didn't like how maxed out units would be basically untouchable since they would mow down anything they saw.
My system makes some changes to address these issues, not perfect but I think it has more interesting decision making. Reaction fire is no longer a roll, during the enemy turn as long as your unit has direct line of sight on an enemy, they start filling a 'reaction meter' (3rd bar below hp and TUs), when the meter fills they either snap fire or burst, chosen at random. The speed the bar fills is proportional to your reaction stat, and a bonus depending on weapon (SMGs and shotguns react quicker than ARs, the LMG cannot react at all).
Once a unit has reacted once the reaction meter takes half as long to refill, to represent your guy actively enterring combat, the initial slow reaction lets enemies get a bit of a drop on the squad, it's always better to shoot on your turn than to wait for reactions (unlike original xcom). At the start of the enemy turn, units also gain or lose TUs proportional to their reflex bonus, this means low reflex units sometimes cannot react at all, and high reflex units will have a pool of TUs even if you exhaust them entirely.
This lets you be a bit more aggressive with both types of units, mitigating the x-com crawl somewhat. Units with middling reflexes fit somewhere in between and behave more like x-com units, where preserving TUs for reactions is more optimal, I don't think the play pattern in x-com is entirely bad, I just wanted to softly disencourage the pathologically unfun case of inching forward 1 tile per turn.
I think there's still lots of room for iteration but I was relatively happy with how it felt.
P.S. Nothing wrong with ignoring the manual, I expect some people to do that no matter what, ideally I would have some better tutorialisation and ingame resources to understand the mechanics rather than using the crutch of a readme :)
Thanks for your input!!