First, it’s your table, your rules are more than fine, specially if everyone is having fun!!!
But as written, (and I don’t think anyone plays ICRPG as writen…but it is a very decent place to start from.)
A Nat 20 gives an ultimate…but a recovery is 1+con bonus, regardless of roll…perhaps 2 on a nat 20??? if you want?
Master Archer +19 on attacks with bow…why add easy to misses???
Endless ammo…cause I don’t want to track it.
For the most part, I suggest making monsters as simple as possible so the players can fight 20 slightly different things and you don’t really need to bog yourself down with ‘what 9 guys missed last round?’
Target Number= enemy AC and defenses.
Their HP and +'s are the only things worth noting most of the time?
Loot is generated differently, so it does not matter who has what…except if you want that guy to drop specific loot for story purposes. Swords, bows, clubs, axes do D6+effort bonus, spells do D8+effort bonus…
As a GM you roll against PC Armor. and you may roll 40 times a round…against different TNs since you are using PC armor not TN.
If you want a special NPC you can do whatever with them…but why complicate something that you have to multiply by 20 at times?
when NPCs roll Nat 1s fail…sometimes critically…more importantly it is a descriptor so the Players learn how evil you can be.
when NPCs roll Nat 20s add a d12 to effort to balance out against PCs.