Updating an old iMac by adding a mini Mac: most cost-effective option? How to set up?

I have a 21.5", 2017 iMac certified rebuild that has reached its limit for updates at Ventura 13.6.7. It has a 2.3 GHz Dual-Core Intel Core i5 processor w/8 Gb of memory. I make use of a subscription to MSOffice365 and use Apple apps for work and home respectively (self-publishing, photo-video presentations, spreadsheets, streaming, internet). I also use Google apps quite a bit. Since adding Office, responsiveness is painfully slow (lots of 'beachballs'). My screen is starting to black out occasionally requiring a restart so it's time to update. Looking for most cost-effective fixes. Considering buying a miniMac with maybe a second-hand monitor--or both could use existing iMac display using screen sharing, I think. Could I run Apple updates on the newer miniMac loaded with Apple apps and use my iMac mostly for Google apps and MSOffice? The goal is to be able to keep Apple OS updated and seamlessly transition between Office, Google and Apple apps and keep multiple programs running. Teleport was recommended to control multiple Macs using a single keyboard and mouse. Two monitors are nice but not necessary. What's my most cost-effective configuration that would actually work?