I had the same experience of coming back to Dark Souls in late game after a long break, it was tough but it's doable. I've only beaten the game once so I'm not an expert or anything but I'm gonna try to give you some advice.
Basically the way it works is, after you've upgraded a weapon to a certain level like +5, you need to
ascend it before you can upgrade it more. You do that in the "modify equipment" menu at a blacksmith. This menu is also where you can switch to a different upgrade path like crystal or divine, but if you want to get the best dex or strength scaling it's generally best to stick with normal path(crystal scales better but breaks).
If you got this far in the game without ever upgrading past +5, that's actually impressive lol, and you're gonna notice a huge difference in your damage output once you upgrade your weapon further and it's gonna make the game a lot easier.
As for the catacombs, yes you can kill the skeletons permanently with a divine weapon, but you can also just kill the necromancers who are reviving them. Whenever you kill one, the skeletons in their area won't get back up after killing them, and iirc the necromancers don't respawn.
You can only summon people when you're human. Use a humanity to reverse hollowing at any bonfire to become human. When you're human and in an area where the boss is still alive, you can see other people's summoning signs. They're usually around bonfires and right before bosses. You can also summon NPCs if you find their signs in specific places. If you got the white soapstone from Solaire you can put down your own summoning sign and be summoned by other people, help them kill a boss and you get souls and humanity. You don't need to be human for that.
btw I get that you don't want to look things up so you don't get spoiled but definitely look up how to access the DLC before you finish the game because it's kinda ridiculous and really easy to miss.
Hope this helps, good luck!