This is how I'd compare 2k21 to TGC2019 in regards to a mouse (note: this is after the latest patch update).
The Pro-Am swing difficulty in 2k21 is somewhere in between Pro and Masters clubs on '19.
The Pro difficulty in 2k21 is more like Masters in '19 ... but just a tad harder.
The Masters difficultly in 2k21 is probably 2x to 3x harder than Masters in '19.
Online matchmaking is locked to the Pro difficulty level (so everyone is on an even playing field - which is much better than '19).
Societies are able to lock their events to specific settings/difficulties ... so, there will likely be a lot of variety out there.
As for the mouse swing itself ... the swing needed to hit "perfect" tempo in 2k21 is a bit faster than the swing in '19. To me, it feels like it did when I tried to deliberately hit a 95% shot in '19 (starting your downswing just a split second before reaching the top of your backswing).
A week ago, I would have advised that you not waste your money (yet) until things were fixed ... very happy to say they got out some fixes pretty quickly, and everything has gotten a lot better (yet, again, still noticeably more challenging than '19). I guess it all depends on what level you wish to play at and how much you want to be challenged. Regardless, I think there's a lot more choices out there now of where to play.