Short of hours? Rent. Or CFI on the side at the local airport. Or GI bill the multi engine license. That would get you a chunk of the 12 hrs.
Go reserves and get your 20 yrs.
Get on with a regional ASAP.
You can apply to any company while working for someone else.
Helo time is typically valued at zero for the Big 4. I think JB and Spirit count it towards their requirements. You'll get a bump for service and aviation training perhaps but the time is typically given a zero, or close to that. You'll get credit for additional qualifications of Stan/Eval, IP, Safety, etc. So most of your flight hours for competitive purposes will come from the civilian flying.
You'll separate at 31. Civilian wise you'll be pushing 3000 hrs at 35, 5,000 at 38. That might be the sweet spot. But requirements might lower in the 6-9 years I mentioned as the retirement bubble expands.