ISSA (based out of VA) is very similar to SSUSA with a few differences.
4GG for entry fee
0-0 count (no foul)
only need a license to verify address (MUST in state or border)
team registration is $15 per year
balls are AD Tattoo (hotter than SSUSA IMO)

Major is 6HR allowed with only 5 runs per inning, same as SSUSA

Depending on the time of year:
Senior Cup in Roanoke, VA end of June is a very nice tourney, great area and a couple of VERY cool complexes (Botetourt and Moyer)
Worlds in Richmond in early Aug teams
The open end of Oct in Myrtle Beach is very nice, turf fields all 325

You will 6 - 8 55M teams in most of these events

Website if you want to check it out:
https://www.seniorsoftball.org/

Hope this helps