If you are reliably logging 70-90 hours a month with "a lot of that multi time" stay right where you are. That puts you only 10 months away from ATP numbers and a regional job. Any change at this point will only slow you down due to transition training alone if nothing else. Retirements at the legacies are already ramping up but will largely peak by 2023. You can either just beat the wave, ride the wave, or be behind the wave. Getting to a regional as soon as possible gives you a chance to do the former. Getting a 300 - 500 hour a year corporate job and waiting a couple years to upgrade will practically guarantee you miss the wave. A year of 135 multi turbine and you'll STILL be starting out as an FO at a regional to build a 121 resume and won't upgrade to PIC a day sooner so - from a getting to your goal standpoint - is a little better than treading water, since at least you'll be getting turbine time. You'll still almost certainly need TPIC from a regional.
Your choice.