Hi Suzy, I have read through all the posts to your original query.
You do have another option. Namely to continue using what you have until it wears out and stops working. In that regard, anyone using an older Mac should always have one and preferably more than one good, verified backup. Backups can be verified by restoring selected files and folders now and then. All electronic devices do fail at some point, it's a matter of when not if.
It sounds like your existing Mac serves your needs perfectly. If you wait longer to replace it, you will be getting a better new Mac because they get faster and better every year.
I am writing this on a 2015 iMac running Monterey, which is MacOS 12.7.2. This is also an "obsolete" computer but these Macs are still quite secure. Assuming you are on a network behind a good router with normal security turned on and good passwords, and you have the Mac firewall on, I think the chances of you being infiltrated are nil, much lower than the risk of a catastrophic automobile accident every time you drive your car around the block, yet you don't worry much about that do you? By the way, I have newer Macs running Sonoma with all the latest, but I like this 2015 iMac as it has a great display and is more comfortable to work at than the other Macs I have which are laptops.
If you do end up getting a new Mac (which you really don't need to do at this point), I would keep it simple and get either a new iMac, in which case don't skimp on the specifications because like me, it sounds like you use your computers for ~ 10 years or more, so make sure it is well equipped from the get go, or a well equipped refurbished one with a good warranty which you can get from either Apple directly or OWC MacSales. These two sources have a great reputation for refurbished Macs. A local Apple Authorized Service Provider (third party store) may also offer good used Mac deals with warranties and also help you with the "conversion" to a newer Mac.
But after reading carefully your questions and concerns, it sounds like you really don't need to switch to a new Mac right now. Because what you have is working well for you!
By the way, with a new iMac you may find it takes some effort to continue using your older printers, scanners, etc. I am familiar with this and as an engineer, am quite comfortable dealing with it, I am even still using a 2011 Canon printer/scanner and a 2014 Epson printer/scanner. But to get all their functions working under Sonoma required some workarounds were needed. My wife had been using the 2015 iMac and I set her up with a new M1 iMac last year. I had to scurry around and do a bit of work to make EVERYTHING look just like it used to for her, and now she is thrilled with the new(er) computer but I had to go to some effort to make it seamless. (She is still annoyed that the old pushbutton scan function on the 2011 Canon printer/scanner no longer works with Sonoma, she has to scan with that device in a different manner now and she doesn't like that change.)
None of the reasons you mention for getting a new Mac sound compelling to me. Personally, I always welcome finding reasons to buy a new Mac, and am considering one of the new laptops now, there is nothing more fun than a new Mac. But if you have financial constraints, and what you have works great for you right now, I don't see the need to disrupt your life at this point with a new purchase and conversion of all the peripherals.
By the way, your iMac has fast connections/ports, gigabit ethernet and 802.11n WiFi which are not bad at all. So it is by no means outdated in those respects. To get the best security you can (optionally) install Catalina 10.15.7 (check that your Microsoft software is compatible).
Just a maybe contrarian perspective, but as someone who still has good working Macs dating from 2010, 2013, and 2015, as well as the newest and latest, you really don't HAVE to change to a new Mac at this point.
