2019 iMac very slow

I bought this iMac as an upgrade to replace my old 2009 iMac. Right out of the box it was slow. Takes ages to boot up, log in and open apps (often 4 to 5 minutes from switching on to being able to start work), frequently get spinning coloured disc. Spent the 1st few weeks back and forth with help desk. first theory was that there was an issue from the Time Machine back-up from my old Mac that I had loaded. So wiped the drive clean and individually installed all apps and manually copied over all data files. Still the same but by now too late to take it back. Then we got to COVID-19 lock-down so unable to do anything about it. When got going again help desk came to the conclusion that it must be a hardware issue. Got booked in to local approved repairer and took it along. They tested it and said no problem with hardware. They also said that this model with the hard drive always runs like this on Catalina.

This was a real shock. After spending £1300 to I was not expecting it to be slower than my 10 year old machine. I eventually got an appointment to see someone in the Apple Store where I got it from and they basically said tough - that's just how it is and they couldn't help.

I had read that loading the system onto a solid state drive and just using the Mac's hard disc for data storage should fix the problem. I have installed Catalina on a 64GB flash drive and booted from it. This was slightly quicker to load and run things but still slow and still lots of spinning coloured disc so after trying this for a while I have reverted back to the original set up.

I have upgraded to Big Sur in the hope that this would fix the issue but if anything it's worse.

Apart from trading in for a better Mac what can I do fix this? Some days it is almost unusable with the slow response times. I have attached the Etrecheck report.

Really disappointed with Apple on this one. I explained to the guy in the shop when I bought it that I was upgrading from an old iMac because that was getting too slow and I wanted something faster. He never said that the one he was selling me would be slower. Then during countless hours with the help desk during the first 2 weeks they never said that this is an issue with this model. If they had I would have taken it back and got the higher spec one!!