Yep, the M is for mb, that's for RAM in this case.
As for levels... That's not really that black and white, IMO, there's no "best" platform, it's always "it depends". I think you're fine with a pi, certainly for a while, and especially at the price point. Plenty of folks running fedi instances and Matrix off of 'em.
The only thing that comes close and has good software support would be a second hand small form factor office PCs, like HP mini PCs or the Lenovo ThinkCentre. Might buy you some expandability down the road, but it's slightly bigger, uses a bit more energy, choices. It depends.
Does getting beaten into a comma, give you pause?
(Couldn't resist, sorry ๐)