Starting June 28th, you will no longer be able to log in to a Curse account that was not merged with a Twitch account. If you have not yet merged your Curse account with a Twitch account, please do so here! Otherwise, your account and its content will be inaccessible.
The best thing to do - is to do what's fun for you!
Levelling up and progressing in your second full season will be easier. If you play casually, then there will be plenty of people on these forums who will be very happy to power level you. That way, you don't have to go through the grind of levelling up to 70 (if you find it tedious). Effectively, you will catch up with your previous P180 reasonably quickly, so will not really be missing out. You can also dip in and out of playing the season/non-season toon - so best of both worlds :-)