BLUF: Title, basically.
Extra Context: A couple years back, I picked up the 4th edition of Twilight: 2000. For those don't know, this is a post-apocalyptic RPG which was first released during the absolute peak of Reagan-era Cold War saber rattling. Unlike other, more fanciful or escapist takes on life "after the end", Twilight skewed much more simulationist, with Tom Clancy levels of hard-on for NATO and Soviet personnel and equipment statistics. Think This War of Mine meets ARMA 3, moreso than Fallout or Wasteland.
I start with this preamble to communicate how different in tone this system is from the games of 5e DnD I've played and run in the past. While the latest version of the Twilight rules is, to my understanding, significantly less packed with grognard crunch than previous iterations, I still find myself overwhelmed with all of the various mechanics and slight rule variations. As an example I hope is illustrative, to fire a weapon classified as a machine gun, your character uses their "heavy weapons" skill, but they follow the "ranged combat" rules, because heavy weapons typically refers to explosive type weapons. You also have to remember whether the MG you're using is classified as Light, Medium, or Heavy, as there are different requirements/penalties based on your character's stance. In and of itself, not too bad, and it pretty much tracks with the considerations I would put in place if I were designing this system, but it becomes a bit much with the everything else of the system. Case in point: the entry for hand grenades states: "grenades use the same rules as heavy weapons, with these exceptions:" and then goes on to list 6 bullet points, which makes me think that it hardly uses the same rules as heavy weapons at all lol.
To be clear, I don't necessarily want to jettison this level of crunch, as I think it's integral to the tone the system is designed around, but fuck me if it isn't a lot to keep track of, especially since I'll have to shepherd potential players through it efficiently if I have any hope of actually playing a game.
With that being said, any advice from the field? Experience with Twilight would be a bonus (any edition), but if anyone has general tips for tackling new systems, I'm all ears.
Allow me to disagree with the highlighted statement. A mistake old school game designers used to make is to try and reflect every fluff detail in the rules. You don't have to differentiate an assault rifle in full auto from an SMG and in most cases, even an HMG. If you go down this route, you will need to account environmental factors, fatigue, time of day... which are also big influencers. But we don't, because that's what randomizers are for. Just bundle the difference in the randomizers.
You'll say, correctly, "getting hit by a .50 cal is a world of difference compared to a 9mm". That's true. That's fiction and setting, though. It doesn't have to be reflected in the rules of simulating shooting the things in a game.
My advice would be to do exactly what you proposed not to: chuck the entire system. Use a generic system instead. Whatever you're most familiar with: Cortex, Savage Worlds, Fate, GURPS...
What is your story about? If it is about handling the gear, then perhaps (perhaps!) keep the OG system. If it's about anything else - a shared overarching plot you'll write with the players; about how the characters react and evolve to their surroundings; about interpersonal drama; base building, you name it - use a different system.
Of the ones I listed, Savage Worlds is easily reskinned, Fate is easily tweaked, Cortex is a toolbox to get your game moving fast just the way you want it to feel, and GURPS probably already has a module for you to use. If you want to bounce ideas with other people adapting to Fate and Cortex, they both have very active Discord communities (I know, information silo, but that's where the authors hang out). Good luck!
P.S.: I just remembered Cepheus (a Traveller retrocolone) has the acclaimed Modern War from Zozer Games. I have played their Hostile (Alien universe RPG) and the ruleset is essentially the same. There is some crunch, but easier than D20: https://www.zozergames.com/modern-war.html
It looks like it and Delta Green lead sales on military RPGs: https://www.drivethrurpg.com/en/browse?genre=320-military
I personally disagree with the assertion that simulation is unnecessary because dice exist. Crunch and randomization work differently, and serve different purposes in a design. Crunch is good for accounting for the factors that are known, predictable, or obviously true in the fiction. Randomization, by contrast, is good for things not easily known or tracked, or that one could never guess would be true in the moment.
The point is not that crunchy simulation is undesirable, but that every simulation model is an exercise in where to draw the line in what accuracy you're losing to keep your goals achievable. OP has expressed the crunch is too much for them. "It's okay to draw the line further away from accuracy" was the core message I was trying to convey.