Random sleeping, problems with battery... SMC won’t reset

My Mac has been working fine for at least a year or 2, not many times I have let my Mac unplugged and sleeping for a while but one time I let it run down well below out of battery and it didn’t start up until it was fully charged. This problem persisted every time it kept getting run down to zero. I let it charge for a few nights and did an SMC reset which did the trick for a while.

After the Catalina update I realised that I couldn’t run 32 bit applications anymore so I downgraded back to Mojave using a bootable drive and just reset everything and started from scratch. After a day or two it started to just shut down or sleep whenever it wanted even when it was charging or I was working… SMC resets stopped working, LED’s on the AC didn’t show it reset and none of the power settings reset. Resetting the PRAM worked but the SMC didn’t. The problem keeps persisting and I’m hesitant to take it to a store.

Any answers on why this is happening? I’ve ruled out the battery is healthy, no liquid damage, it won’t let me reset SMC, charging and charge ports are fine but it keeps sleeping every 2 mins after startup.