The only gen 8 format where you can use Dynamax is AG and National Dex AG. Both of these formats are riddled not only with strong physical attacks, but also strong fighting attacks. Snorlax already has a very hard time not being 2HKOed or flat-out OHKOed by Close Combat from Aegislash, Cobalion, Conkeldurr, Doublade, Haxorus, Incineroar, Lycanroc-Dusk and Hawlucha and Superpower from Darmanitan, mixed Hydreigon, Mamoswine, the list just goes on and on. If you that that was bad, OU is a absolute nightmare for Snorlax, as as it struggles to keep up with Urshifu, Rillaboom, and other threats that often carry Fighting-type moves.
"Later you could sweep with only this Pokemon, yeah, I'm not freaken kidding you." A sweeper by definition is a Pokemon who is fast and strong. Snorlax has base 30 speed, and yeah in theory it can sweep for 3 turns, but what happens after that? Also you probably shouldn't use Toxapex as an example, since Snorlax needs set up to work or it is pretty useless (unless Choice Banded), so Toxapex can just Haze and make Snorlax waste its HP stat and a berry.