iMac running awful! Very slow, lagging, unwanted tabs in Chrome etc

I'm at the end of my tether with my late 2013 iMac. I've reported this same issue for over a year now and was advised to get more RAM - I did, to remove adware- I did... everything I've done has made no difference and I'm honestly considering a full erase and start again but that'a major hassle and work I haven't got time for. Here's some of the issues off top of my head (so many!!):

– Chrome keeps adding unwanted tabs, plus after a while the home page changes to something other than Google with no option to change it back apart from another reset. Sometimes a whole bunch of the same URL tab greet me in the morning and that's happened since putting the Mac to sleep!

– Opening new tabs manually makes everything else (like video players on different windows (eg. Eurosport) just freeze, lag, skip etc

– A restart literally takes the machine about 10 minutes to be up and running properly ie. so apps aren't lagging and email messages are preloaded.

– Switching apps sometimes lags, occasionally crashes them.

– Searching for emails in Mail sometimes takes far too long, other times it's fine.

So you can see my frustration, there's probably issues I've missed. If anyone can advise I'd be grateful, otherwise I'm going to have to fully erase, just can't see any other solution.

Thanks

EDIT: sorry I've jst realised I've specified High Sierra when it's Catalina. Not sure if this makes a difference, the same issues were when I had HS anyway!