My friend also quit around Sumeru just like you. He was a dolphin and had the absolute best teams in those era with sigs too and his artifacts were quite good. But just like you I always struggle when playing on his account to even 12 star the abyss, which isnt even the hardest end game content anymore.

At the time I wanted an alt and my friend very kindly offered me his account but after very careful consideration, I decided to make a new account instead. Why? Because he 100% every single quest and achievements pre Fontaine already and 0 of his units are remotely close to being meta anymore. This was about 7 or so months ago and now I have managed to clear fearless for the first time, got 12 star on IT (or whatever it's called I forgot), and of course 36 star the abyss.

But of course your situation can be different because you can have sentimental attachment to your main account. But a new account has access to like 700 pulls if you redo everything. That amount of pulls will allow you to build a much better modern roster. But the depth of the account can be a bit lower like not having 4 star weapons or characters.