I think you could beat it in 20-25h if you're really selective about what you do. As far as side content goes you can probably skip most POIs, but you'll definitely want to clear out all the AA sites and probably any checkpoints you happen to be near.
For weapons, all you need are the SSGP-58 rifle (AP ammo, 5x ACOG and other 4* mods), MG42 LMG (blast ammo, 4* mods), and Compound Bow (EMP arrows, whichever sight you prefer). I'd advise spending the 300 premium credits you get for free on the Cartomancer shotgun, it's easily the best premium gun and there's nothing else worth spending that currency on. If you care about having a pistol, get the Desert Eagle (it's from an easy treasure hunt in Esperanza). If you don't do a lot of side content, you'll be severely lacking in upgrade materials so it's pretty important not to waste any gunpowder or electronics on anything but the best of the best guns. It's fine to buy stuff like ammo, scopes, pointers, suppressors, etc. for early guns because that stuff is useable on all weapons of the same type. Just don't waste mats on garbage like SMGs or sniper rifles.
As far as armor goes, you can complete the game with just the Parkour and Scrounger sets which you find very early on. Prisoner's set which you should find quickly in higher level areas is handy too. If you have the season pass from Ubi+ or whatever, it's definitely worth using at least the chest and gloves from the Blood Dragon set.
For amigos, Guapo is so bad that he's actually a liability. Instead, do the treasure hunt that starts on noob island and then sends you to each of the other 3 regions. It doesn't take long at all and you get some pretty good shit from it, including the only amigo that's really worth using. I'd strongly recommend doing the Chicarron missions too. He's useless, but the missions themselves are some of the funniest writing in the game imo.
That should be about all you really need to do outside of the main story missions.