Most of the SLA problems are people that use a standard Power wheels charger. They kill batteries quickly due to no float = constant charging for days damages the cells and low charging amperage. However a decent 2-3a 12v charger ($20) will extend the life of your SLA batteries and charge much faster. a 12v 12ah SLA is only 25$ and generally gets an hour run time +. I have a few that last 2+ years so far.

I'd say just buy a 20v dewalt adapter instead of chopping up a tool https://www.amazon.com/TOMAPEX-Battery-Converter-Adapter-Source/dp/B08P1VNC9C/ref=sr_1_1_sspa?dchild=1&keywords=20v+dewalt+adapter&qid=1630339830&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFEMU5EMzVERlQzUVomZW5jcnlwdGVkSWQ9QTA1NTcwNTQyVVJXREI2RzhNUFg0JmVuY3J5cHRlZEFkSWQ9QTA4NDA1NzIzREI0M05DMUFYMEEzJndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

I'd run atleast 2 in parallel so you get decent runtime, Preferably 4-6ah batteries. The lower 2ah batteries don't get much time and get hot quick from the high discharge rate.