The Air Force is facing a massive recapitalization and expansion requirement over the next several years to a decade, so even outside of B-21 you need to really expand the procurement budget to get things in order.
The Air Force has to start replacing a large portion of its current manned fighter fleet otherwise I suspect the airframes are just going to get too old to maintain even with upgrades. The current average age of the Fighter / Attack fleet (A-10, F-15 C/D, F-15E, F-16 C/D, F-22A and F-35A) is ~27 years. I am not sure how this would be done without a drastic increase in the procurement budget of the Air Force. Outside of that, you would likely need to delay foreign military sales to prioritize recapitalization within current production rates.
If you were to retire all existing A-10, F-15 C/D, F-16 C/Ds by say 2033 you would be retiring 1280 aircraft. In theory, with an increased procurement budget and deferral of foreign military sale aircraft deliveries it may be possible to replace those aircraft with a combination of F-15 EX, F-16V and F-35A. Given current production rates, the majority of those would be F-35A, then F-16 followed by F-15 EX. So even though you would be looking at building around ~210 aircraft a year, you would end up with a fleet of roughly the same size as the current Air Force fleet, but with a much younger airframe age and more modern technology.
One potential benefit I see of the Air Force divesting these legacy manned 4th gen platforms in a short time frame is that you could potentially turn the recently retired F-16 C/D into loyal wingman drones. The F-16 C/D would be very capable in that role and should only require minor software and hardware changes to become loyal wingman once that technology is available. I believe many of the existing F-16 have AESA radars and support for numerous weapons and targeting pods. On top of that, the age of the airframe will be less relevant since a loyal wingman doesn't need to maintain any flight hours for proficiency outside of a small number of test aircraft.
Concurrently, you would need an NGAD Penetrating Counter Air platform to be designed and procured. This couldn't been in small numbers, as China already has a large number of J-20s and has recently unveiled the J-36. So something around 50 aircraft per year at a minimum and around 400 airframes if not more.
Continued production of F-35 passed the late-2030s doesn't really seem to make sense in light of these developments either. The plane has a troubled past and was unfortunately compromised by requirements to meet the need of three different services. The Air Force will likely need a new single engine fighter purpose built for Air Force requirements in the Pacific theater.
Really, if you start to think about it the Air Force is facing a monumental task in recapitalizing and expanding its fleet of manned and unmanned fighter aircraft, let alone building a new strategic bomber force with the B-21 as its centerpiece. Just think of pulling off everything above in the current political and budget environment...
EDIT: I will say that while the Air Force situation is pretty dire the Navy's situation is seems much better. They continually produced and procured F-18 E/F Super Hornets over the past 24 years on top of EA-18G Growlers. So they have a large fleet (416 F-18 E/F and 152 EA-18) of 568 airframes that maintain high mission readiness rate. On top of that, China doesn't really possess a naval airfleet to speak of, so procurement of the F/A-XX can be relatively slow in while still maintaining an advantage.