I'm reaaally out of the loop when it comes to building PCs, so I wouldn't dare give you answers

The last time I've built a PC was 2018, and I was no pro then either.
The one thing that I know is really important for BG3 is having the game installed on a SSD and not a HDD. Luckily for me my motherboard kind of supported a M2 SSD and that made everything in BG3 load faster. Still, I can't promise you that by moving BG3 to a SSD on your computer it your issue would be fixed. I somehow get the idea that your PC wasn't a super good gaming PC even when the Vega 8 videocard was released (2018).
It might be best to start with a fresh computer. I have looked into upgrading my current PC (bought in 2018), and it's just not worth it. All the new technologies would be held back by my motherboard, M2 SSDs have become so cheap that regular SSDs aren't worth it, standards have changed for cables so I would likely need to buy either a new power source or adapter cables, etc. I bought two of the cheapest fans I could find and they need 4-pin connectors instead of 3-pin now, I did not expect that and neither did my power supply unit

Components for older motherboards just cost a lot - CPUs and RAM that are comptabile with my current motherboard are more expensive than much better components that work with a new motherboard, and the price difference is half of a new motherboard
