The caches part is correct, just open them and move on
Luckily with the world tier system, any activity except incursions is fine soloable as you can run it on hard until you get the relevant gear to move onto challenge. Or what snips said is also cool, 30 mins in wt4 with a group will get you good gear even just to throw on.
In terms of builds, 3/3 setups are good, as are 3,2,1. For the 3,3 you would want to aim for complimenting gear set bonuses - for example a big one atm is banshee + lonestar which gives, Additional weapon damage plus out of cover damage, or preds, banshee for the same out of cover plus assault rifle and smg damage.
3,2,1 is also cool as it overs utility, gearset, and a modifier. So for example, 3 preds, 2 nomad for health on kill + savage gloves/spec backpack/any HE tbh!!
There are lots of "meta" videos which are good but the good thing about the drop rate is experimentation

you could easily run all high end gear and do everything, or run a full 4 piece gearset like predators/alphabridge(very strong) or even firecrest for the crowd control.
The only sort of "meta" thing you do need to pay attention to is armor unfortunately. Its best in slot for chests, knees, backpacks, bolster and gear mods for PVP - which max rolled native plus major will give 60% roughly mitigation.
For PVE, 50%-55% is sufficient meaning one of those gearset pieces could be rolled to skill power or whatever you want giving more flexibility.
It is so easy to spend hours tinkering with builds, checking stats to get the most out of what you have, which you may also find fun like i do lol.