I agree with #2 and #17 above all others. With money tight, not sure even a $500 Mac Mini is within reach. So get yourself a cheap- but new- Windows box or install Windows 10 on a bootcamp partition on that one to make it into a "new" Windows machine. Either will be able to do all you need to do with secure software and no hacks.
I would NOT buy some old Mac from more than 3-4 or so years ago, as you'll just bump into the same problem in only a few more years. Most of us seem to believe that we could be in the final window of Intel Mac updates, so no need buying a new (used) Intel Mac at this stage.
Use the Windows computer or bootcamp Windows until you can save up for an actual new (silicon) Mac. A new Mac Mini will get you a fresh window of about 7 years of macOS updates until vintaging + maybe 2 years of security updates beyond that. Subtract years from that if you opt for a used Mac. For example, if 2020 and it's basically 2024, subtract 4 years from the 7-9 for a net of 3-5 years before it would probably be in about the same boat as now.
If you are an "Apple/Mac or bust" kind of guy, #10 offers best options: use phone/tablet for now and save up for a base (but brand new/new refurb) Mac Mini. That should buy you up to 6-7 + 2 years MAX useful life, without leaning on hacks.