Lots of variables to consider and many intangible that can't be put on a power point slide.
We have had a few people consider this in our unit over the years. Basically they would choose not to retire and to serve as traditional reservists who already had earned a 20 year active duty retirement. These were AGRs who lived local, were not sure about airlines, airlines were not hiring yet, did not want to take another assignment and uproot families, wanted to stay current in case they wanted to do the airline thing, and the on/off again return to fly as a retiree process was either off or taking over a year to accomplish.
You are correct in thinking that it takes about 12 days of work doing some combo of active duty pay status and inactive duty (UTAs/AFTPs) to earn what you would earn in retirement pay.
Questions to consider
Are you an O-5 or an O-4? If you are an O-4 with PME complete, there is a fairly good chance of getting promoted in the ARC. When you eventually retire from the ANG/AFRC, you would draw a retirement as an O-4 immediately that would include your 20 years active, plus whatever participation points you did as a guardsmen/reservist, then at age 60 it would recalculate to a full O-5 retirement. This assumes you did 3 years time in grade after promotion as a traditional. Of course, if you did 3 years of total active duty time in the higher pay grade before retirement, you would immediately draw an O-5 retirement.
Can you get hired by a regional and go to training while on terminal leave? If so then you won't really lose any money by not retiring. If you can't get hired before you separate, then the 2-3 months of training at a regional to get to the line won't allow much time for doing much other than a drill period or two either on UTA weekend or as makeups. On the flip side, if you don't get hired at a regional before separating, going TDY to the the school house will provide you at least 3 months of pay and you will start building hours sooner. Those hours combined with the network of airline folks in the unit may allow you to skip the regionals entirely.
Is a unit going to hire you, where are they located, is it somewhere you want to stay and put down roots after you get a job at a major? If so, then it might be worth it? The ability to mil drop to adjust your schedule to be home for key events until seniority allows you to shedule your life the way you want would be a QOL enhancer while on first year pay/schedules at a regional.
What unit/airframe are you trying to get in? What is the deployment/tdy rate? Are they going to want a commitment in terms of min days per month participation or number of deployments you have to "volunteer" for? As someone stated above the ANG/AFRC is only getting worse. Trickle down queep from active duty and the PC monster is alive and well. When I first left the ANG in 98 to go to the 340th FTG in AFRC my observation then was the AFRC was about 5 years behind the active AF in terms of queep, and that the ANG was about 5 years behind the AFRC. Since 9/11, OEF, and OIF, I think both components of the ARC have closed the queep gap at an exponential rate.
If a unit is short enough on manning, then hiring a guy who already has a retirement in the bank should not be too big of an issue. You shouldn't be taking away a spot on the manning doc that a traditional already established in the unit might need for promotion. Sanctuary waivers for active duty orders are no issue since you already have a 20 year active duty retirement. The only issue may be a training commitment after spending money on you to get you qualified to fly again since someone else stated above that you could basically walk away as soon as your BS meter pegged.
The pros to doing this are getting both 121 time as SIC and logging more military PIC time in your MWS. The network of airline bubbas in the unit combined with your 121 time will help you land at a major carrier sooner.
The cons to doing this are your life will be extrememely complicated and busy in the short term. Getting up to speed again in your MWS (assumes you go to unit with your prior MWS) while learning a new but relatively easy way of 121 ops, plus the 121 airplane will be a pain in the butt and take you longer to comfortable in both planes. I definitely would not move for a regional unless it was to a place near one of your top choices of major carriers and in a place your family wants to put down roots.
Good luck, lots to consider. It can be a good idea under the right circumstances, but it will take a lot of research to pull it off. In the long run, it would be worth it, but it will suck regardless in the short term.
Feel free to PM me if you want to discuss any ideas.