I don't have a Macbook but I've bought refurbished Apple products before usually from the Apple refurb site simply because of the new battery and shell/case - I've bought a couple of ipads - a 12.9" one that is a 2017 model - I bought that in 2020 and it's still fine with it's refurbished battery.
Do you actually need to use it off battery a lot or for long periods of time?
You can send it to Apple to get a battery refurb done - I know there are plenty of other places that do it that are probably cheaper than Apple. I bought an ipad 3-4 years old without a refurbished battery (not from Apple) and the battery life was deteriorating after about 3 years - it was too old to get the latest iOS updates too so I bought another (from Apple) - I know - different device to a Macbook but I understand your concern about battery life.
If you go to
https://support.apple.com/en-gb/mac/repair
you can select a model and get an estimate for an Apple battery refurb.
I selected am early 2020 13" model and the quote was £245. UNder their Apple+ care insurance - if the battery capacity drops below 80% inside the year they replace it for free so 80% battery life is, I guess, around the kind of figure you should be looking for. I don't think the Apple refurb store will offer many, if any, refurb items than are more than about 3 years old.
I've bought a lot of laptops and a couple of ipads over the last 20 years from
https://www.morgancomputers.co.uk/search.php?search_query=macbook§ion=content
they don't have much in stock ATM but maybe worth emailing to ask if they think they will be getting any 2020 models in anytime soon.
A lot of musicians use Macbooks in order to use a popular multitrack recording and mixing software "Logic Pro" so a lot of MacBooks pop up for sale on various musicians websites that sell other things in addition to used musical instruments.
I've bought a MacMini with an M1 processor as an alternative to a MacBook (in order to use Logic Pro) because I wanted to use it with a huge 27" monitor I have. It might be worth keeping an eye on this site for instance:
https://www.basschat.co.uk/forum/88-sell-anything/
scroll through and you'll see Macbooks and MacMini's popping up now and then. You'll probably have to create an account (free) to see the postings. I've bought a few things on that site and been on it for years - they are a good lot of helpful and reliable people in my experience.
Like me - do you actually need the portability of a MacBook Pro or Air? A MacMini and a separate monitor was cheaper overall for me and it was what I wanted - no need for portability and I needed an abnormally large screen.