Wife has a 13" MBP 2012 model (build date is 2015) that's getting long-in-the-tooth now. Today I thought about how to find a replacement for it, and that there is no immediate "no brainer" choice. Going back 3-5 years and they're all butterfly keyboard+cooling fan (ie suck dust into the keyboard frame) except the MacBook Retina 12", that's nice (I did like mine), but No. (Simply way too fragile.) My guess is the 2020 MacBook Air with i3 processor. Is there anything else that is even a contender?
State your budget concerns.
Looking to buy old to save money probably isn't such a good idea now that the entire Mac lineup is essential ARM now.
Your option is to stay stuck in the past by upgrading internals in what you have now, buying an old INTEL model or getting a new ARM Mac.
Long-in-the-tooth means you're already operating in the past and have done so for quite some time. So not buying an ARM Mac is still certainly viable if you wish to continue in that state.
However, if your itch to replace what you have now is to allow you to take advantage of what Apple has to offer in 2022 going forward, then ARM is the only way to go. Eventually, you either end up weening yourself off the computer entirely (which many in retirement years tend to do), or you find yourself wanting to do more to do with it... which means you have to at least have a new computer to position yourself going forward.
Apple computers are certainly not cheap. They never have been cheap. The reality is you are at a major transition point. If you have used Macs for any length of time, you would have gone through two major transition points thus far... the switch from OS9 and the switch from PPC. ARM is yet another major transition point. The question is, which side of that transition do you want to be on? There are plenty of folks who continue to use older Macs, but they have no desire to buy a new Mac either.
The one absolute here, that I can see, is you are not in an immediate need to replace anything. Take your time and figure out where you see yourself in say 10 years from now in terms of computer usage. It's funny, but I spent my entire working life on a computer and now that I am heading for retirement, I have less and less desire to use the computer at all. On the flip side, my mother never spent a day on a computer until she was retired. After her stroke, she never could use a computer again. So life does factor into the equation.
Most purchases are impulse buys with little consideration for down the road in all reality. They say things like "future proof" and all, but in the end it's more of an immediate warm fuzzy that they are truly after. They're not likely to hold onto a computer for 10 years or more... thus "future proof" is moot.