I would test the cold cranking amps on the battery to see what percentage it holds compared to what it is rated for.

Also, after putting that load on the battery you might have to wait a while for the voltage reading to rise and to be accurate.

Good advice! I always do that if a battery is suspect. But you do have to get the battery fully charged first (or at least full as it will take in say 12 hours with a decent charger.

I used to only have those basic Sears and other chargers. I then bought a marine smart charger and haven't looked back. It is marine version of this one that they now sell here in Canada I am sure there are equivalent units available in USA.

Basically what I am trying to say - Get a decent charger! Get the battery as fully charged as you can, then do a load test with a cheap unit like this.
https://www.schumacherelectric.com/products/battery-load-tester/
Or maybe one of the more sophisticated electronic units! They were out of my price range, but looked neat
https://canadianbestseller.com/pd/ancel-bst500-12v-24v-100-2000-cca-automotive-battery-load-tester-car-cranking-and-charging-system-analyzer-scan-tool-with-printer-for-heavy-duty-trucks-cars-motorcycles-and-more/?v=3e8d115eb4b3#gsc.tab=0

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5