At least Iso will import our previous levels, right? If I have to do that entire level 30 grind again...
That isn't how the ratings work! It sort of looks like that when the system has zero information, but only for a short period of playing. That's because the iso levels (and the Goko ratings) are their estimation of your skill minus some uncertainty (i.e. it is a conservative estimation of the player's skill). So when you start out it looks like their estimation of your skill is really low, but actually it's just really uncertain. After you lower that uncertainty by playing some games, your changes in level aren't from grinding out games but from actually getting better (mostly).
So if you're iso level 30, the "grind" will just be playing enough games that your uncertainty drops to a reasonable level. I'd guess that with a fresh account a person would be within 5 levels of their "actual" rating (higher or lower, with lower being more likely due to the uncertainty being subtracted) within 50 games or so. I'm confident that within 100 games your rating would just be whatever it's "supposed" to be (within a level or two).
I am making these numbers up, and probably skewing them low to support my position. But I have on a couple occasions created alts and gotten them to near the top of the leaderboard and it doesn't take that many games. People used to do this on isotropic as well and just by glancing through the current leaderboard you can see lots of players with high ratings and a low number of games. It doesn't take 1000 games to get a good sense of a player's skill.