121 and mil time will get you more points on your apps than 135/91 etc

My opinion- do the regional 121 cap. It's what the legacy carriers want. They want to see you in the seat making the decisions. Kalitta right seat time would be very interesting. Nothing against Kalitta, but I see who gets hired, and its military and regional captains with check airman experience.
Don't apply for any job you are not willing to accept. You burn bridges you might need later. I wouldn't apply to Kalitta and possibly be hired. Then turn them down. You might want to play that card later.

My two cents.
Do K4. Get heavy time and the experience. You'll probably get the 767 with your background, but maybe the 747. I think you will enjoy it and it's a great experience that you can't duplicate with the regionals, but if you don't like it you can always do the move back to direct hire captain. And be a better pilot in the process. Too often I see pilots "pigeonholing" themselves, not branching out and getting the breadth of aviation experiences that might save your bacon some day.
Good luck with your interview and hopefully a decision. This is a good problem to have.

Things are slowing down very quickly at K4. The ACMI industry is no place to be on the bottom of the seniority list. An LCA at a regional is just as likely to get picked up by a major as a Heavy FO (the plane, not their weight) Go to a regional with flow and keep applying externally.

Just want to share an experience I’ve heard before.

Captain I flew with at my regional started at SkyWest, got his 1000 hours but instead of upgrading, he went to one of the cargo carriers (I forget which) and got a 767 type. Did that for a while and applied to Alaska. They said go get PIC time.

Just figured I’d share someone else’s experience.

Assuming equivalent TPIC, yes.

Fastest path to big six is still the regionals, statistically speaking. There are of course plenty of exceptions, and if you think you can stand out and become an exception (or just feel lucky, punk) then pursue your exception of choice. If you want to play the odds go to regionals. If you're hell-bent on UPS/FDX, you might enjoy an advantage coming from ACMI, but I'd talk to to people in the know, that may have changed in recent years.

If you have other considerations, such as prefer ACMI lifestyle, then that changes your equation, might make sense to trade career progression for near term lifestyle, and still end up in the same place eventually.

Of course none of this absolute, no way to know in advance exactly what any individual's timeline will look like, just talking general trends here.

For those chicken littles who think the sky (economy) is falling, that would be a big consideration. You might end up stagnant at a regional, but likely unemployed at an ACMI.

But you have to use your own crystal ball for that, no one else's will work for you.

Don't apply to any job you aren't willing to accept? That would be a direct contradiction to almost every piece of advice ever given in this industry. Interviewing somewhere and giving them the TBNT isn't going to hurt your career progression anywhere, and is done all the time.

Counting them chickens.......regional type indeed........

Generalize much?