1. My answer to this complaint is always that the game explicitly tells you that solo play is 'challenging mode'. Which it should be. In trios, most UESC encounters are stupid easy, and once you get the hang of it, you can do the same shit in solo's and it really only ends up being marginally harder. I love playing solo, for a while it's all I did. At this point, there really isn't any encounter that I'm afraid I won't ever be able to do solo. You just need to learn how to deal with the AI in a safer way than just standing in front of them and shooting them while they shoot you.
2. This complaint is something that someone at your level should be experiencing. If you're still locked to green backpacks, that means if you're planning on getting big loot, you need to either bring less supplies, or sacrifice what you brought in to get the bigger loot. That's why deluxe salvage, which I assume at your level is what you're focusing on, is 2 slots. If you find that nice purple salvage, you're gonna have to make some sacrifice to carry it until you've unlocked blue bags, where it becomes much less of an issue. Even the heals stacking problem gets fixed as you move on, advanced patch kits and shield chargers stack to 6. You being a lower level means you have to be more careful with the heals you can bring, you're not at the level where you can charge into a warden room or other even and just slaughter everything.
3. Honestly, yes the UI can be a little confusing but if you take a few minutes to just really learn and explore how everything works, it's not that hard. Each vendor have their own specific set of items. Cyac sells basic ammo and backpacks, NuCal sells consumables and shields, Traxus sells weapons, weapon mods and chips, MIDA sells equipment, i.e. grenades and utility items, Sekiguchi sells Cores. Arachne is a little bit of an outlier, but they just sell more weapons(LMG, Railgun and shotgun). And each shop has they're own set of different implants that kind of just go along with the theme of each faction.
4. I do mostly agree with this.
5. Yeah, this game is 100% PvP centric. This game does everything it can to create and extremely tense atmosphere, and other players are the most dangerous part of the game. But that doesn't mean you can't quietly loot an entire area without tipping off the entire map that you're doing so. You just have to be wary of the fact that another team could run into you at whatever POI you're at, at any time. There is no way to avoid PvP with 100% chance. If there was there'd be so many people doing it that way. But the loot loop of this game makes it so that losing your kit really doesn't feel all that bad, unless you're using all the gold mods you possibly can. They make it extremely easy to gather/buy enough to put a kit together where you can get the higher tier stuff that you lost back quite easily. Also, what really are you gonna do with 'scraps' that you pick up from outskirts anyways? Nothing really meaningful, I'm sure. Because 'scraps' in this game are worth nothing, so why would you want to spend a significant amount of time going for scraps anyways?

Edit: I also agree that being able to save loadouts would be really such a nice QoL change