Plus once you’ve ran through a companion quest once or twice how many times does it still remain fun.
An awful lot of times - at least as many as the main game. This is obvious from this and other games with companion quests. ME2 remains the most replayable ME game by far, and it's basically constructed from companion quests. All the biggest moments and coolest quests in the body of BG3 are companion-related.
One option I have seen is to use two mods made by the same guy. Fast Level To 6 which means you accelerate the first six levels only, so you leave the beach area at level 6. Secondly there is an level cap remover which means you can level to 20 as long as you multiclass and don’t take a character past level 11 in any single class (level 12 has to be the last level take). This same mod also doubles experience simulating the XP boost from not sharing with others. The mod maker assumes you can get to approx level 18 or 19 with this method.
Why you even bother playing it solo if you're just going to cheat?
Isn't the whole and entire reason for doing a solo run to do it legit? It was with BG1 and BG2. I can kind of see a mod to not split the XP, and thus make it more like tabletop, but to start at level 6? That's just going to kind of make it a cakewalk, like why even bother?
The final fight to me is going to be reaaaaaally tough without some people to help you. I am sure there are some crazy optimizers out there that can do it, but you need to generate a lot of DPS quickly to win.
I presume you mean the fight before the boss? With the boss itself a single level 12 GWM Fighter could definitely output 100% of the DPS needed buffed only with a single Potion of Speed. With a Cloud Giant elixir it would be trivial.
If I was going to do solo I think I'd probably either just do a Lae'zel run, as she's sort of the most plausible character to sort the whole thing out herself and refuse to get help, or go with Durge who might do the same. I suspect I'd probably want to respec a few times rather sticking consistently to one build/class for the whole thing, but if I was sticking to just one it'd probably be either some kind of hyper-optimized Sorlock, or a heavy armour high-HP type like Fighter or Paladin. Cleric or Druid could work but I suspect you'd both die more and need to Long Rest a lot more. I don't think Rogues make much sense because in 80% of cases you can't kill off an enemy without starting a fight, even if you do in one hit and outside the sight cone of the rest of the enemies (it's unclear why). Thief might for the extra bonus action if you wanted to go Sharpshooter hand crossbows (which is probably a good option, DPS-wise).
Really the main thing will be being prepared for every single fight, and not forgetting to use consumables. With just one character, you'll have truly insane amounts of consumables at the ready - potions, elixirs, poisons, grenades, etc.