It's a two-step process:
Step 1: Place fingers of right hand on left wrist (or vice-versa)
Step 2: Do you detect a pulse?
If the answer to Step 2 is "Yes," you will be hired by a Regional. You don't need to worry about "offending" them by applying too early--it's a seller's market right now. Obviously you'll want to put your best foot forward, but I doubt any "rust" you may have will make a military pilot unattractive to the regionals. Apply early & often, that will ensure you can set a class date for a smooth transition (even while on terminal leave). That will be HUGE from a stress perspective. Applying in May for the Fall is plenty of time--my phone rang 48 hours after pressing "send" on AirlineApps (not exaggerating) from not one but two regional carriers.
As a bonus, if you get that part of the process out of the way, you can time your apps to your "target" airline accordingly. I think you've got your priorities straight, I know a LOT of guys (myself included) who went Regional/LCC route and got scooped up by a Legacy sooner than the guys who were waiting for their "destination" airline to call. Of course, I also know guys who were sitting in class at Delta/United/FedEx while on terminal leave, so, luck & timing play enormous roles, too. The one sure thing: you won't get hired if you don't apply.
Good luck!