Problems with Mac after updating 2019 OS (Mojave) with Sequoia

The problems start with Set-up, where the screen goes yellow for some time and the mouse freezes for a while too.

Photos is working after a fashion. But I don't have access to all the albums, just most of them. Some photos I would need to find in the Album which contains 60,000 images

Rapidweaver keeps freezing. Force quit is necessary even after attempting quite simple operations like adding a photo or trying to save a few paragraphs of text to a page in a project.

Chrome and Mail are OK but I need to be getting more from my desktop.

I think I need to invest in a new iMac. (Should replace every 5 years instead of pushing for 7!) But in the meantime has anyone anything to suggest that would give more functionality?

Also, given I have problems with the existing Mac should I think about NOT transferring everything from the Mac I'll be abandoning to the new one? Last thing I want is to buy a new Mac and fund that I have all the same problems!

Any help greatly appreciated.