Is this game worth picking up now?
Probably, if you like Fallout and you can get FO76 for a low price. It's much worse than 3, NV or 4 but it's much better than it was when it was released. The world-building is excellent. The game-making is not. If you've ever used any mods, you'll find the lack of modding in FO76 very annoying because you remain at the whim of whatever restrictions and annoyances Bethesda impose on you. If you liked settlement building in FO4, prepare for epic disappointent and chair-chewing frustration if you try building anything more than a small shed in FO76. But overall it's not absolutely terrible in its own right. It looks bad if you compare it with 3, NV and 4, but it has been improved significantly and there are hopeful signs that the devs at Bethesda have finally been allowed to do some work on trying to make FO76 into the game that people actually wanted as opposed to the badly made, greedy, patronising, downright stupid and occasionaly downright fraudulent Fortnite-wannabee cashgrab that was FO76 at launch. It will remain a greedy cash grab, but they are genuinely trying to turn it into the game that people actually wanted on top of that (and the people who actually make the games are clearly delighted at being allowed to do so).
The widespread and severe criticisms of FO76 at launch were completely justified, but it has been significantly improved and even back then there were chunks of gold amongst the stinking ordure. Wastelanders marks a major change in the design philosophy of the game and that's important because that's what was fundamentally wrong with the game.
Can I just pay it solo with no issues?
Almost entirely without issues. I play it solo. My character is called Soloman1862 because he plays solo - he's a solo man. As far as I can tell, almost all players play solo. Which is unsurprising as Fallout is a solo game franchise that appeals to solo players and it's using a game engine designed for solo play and it's still at heart a solo game. The multiplayer aspect of FO76 is an add-on and it doesn't fit at all well.
If you play solo, the very first thing to do when you reach level 5 is to make sure that pacifist mode is turned on. It's always on until L5. Pacifist mode
should make PvP impossible outside of specific PvP mode ("Nuclear Winter" mode in the play game submenu) and I think one specific event in the standard adventure mode. You can't harm other players and they can't harm you. It almost works. That should be Tod Howard's phrase - "It almost works" rather than "It just works".
I'll list the issues I can think of. None of them are deal-breakers. I've played a lot (>600 hours from launch) and in all that time I have played in a team once for less than 5 minutes solely for the purpose of completing a quest in a team because that's one of the "challenges" that get you in-game currency ("atoms") that can be used to buy useless and insanely overpriced crap in the ingame shop. Outfits you can't use in power armour (which you will be probably be using because it's so much better than unpowered armour unless you have very specific legendary unpowered armour pieces and a very specific build for them and very specific weapons for that build and even then you'll still have vulnerabilities you won't have in power armour) and camp building stuff you can't use because the camp building budget is miniscule. Yay.
1) The gameworld is affected by other players picking stuff up. If you like to explore, look at environmental storytelling and read notes (which most Fallout players do), that's an issue. If you like to find places to scav certain items (e.g. screws for crafting), that's an issue. You can never know if a location lacks lore and resources because it does or because another player picked it up before you got there. Stuff respawns, but not very quickly. Bethesda devs, i.e. the people who actually make the game, are well aware that this is an issue and explicitly refer to it in media about Wastelanders because it's one of the things Wastelanders addresses to some extent. Some locations are instanced per player in Wastelanders (and presumably afterwards, since it's part of the change in design philosophy that I mentioned) partly for that reason. There's already at least one location instanced per player (the pawn shop in Grafton). Surprisingly, Bethesda decided to actually test it before throwing it out there. That's a new thing for them.
2) Griefers exist. Of course they do. They're part of a multiplayer game. There are exploits that allow them to kill players who have pacifist mode on and even without those they can be an annoyance by following you around and attacking you. Even if they don't use exploits to kill you, they're still annoying. Your only option is to server hop by quitting to main menu and going back in again.
3) Other players get in the way. You want to clear a location for XP and loot? Hard luck if another player has been there recently. You get that
a lot towards the end game because by far the best location for it is the Whitespring resort so it's the de facto default location and frequently cleared.
4) Some events require multiple players. Some of those can be done by solo players in the same place (e.g. killing the scorchbeast queen) but some require an organised group with communication between them to do properly. For example, it is possible (though difficult) to solo the Paradise event but unless you have a large and organised party with comms the best you can do is keep 1 of the 3 creatures alive and that makes getting any of the better rewards impossible.
5) Some challenges require multiplayer to varying extents. There are challenges specifically for parties and challenges that require more than one player but which can be organised between solo players (e.g. revive a near-dead player in water).
Nothing major. You can certainly play FO76 solo (and most players do).