I can understand wanting to stick with a developer you're familiar with...no fault there. But a few minutes in an hour long process is...well, kind of trifling, to be frank.
My favorite second developer (my first will always be D-76) is Dektol.
Develop for as many minutes as you added parts of water. (Rule of thumb taught to me by PE).
So if you want to develop for 4 minutes, dilute 1 part Dektol stock to 4 parts water.
It's a rule of thumb, take it with a grain of salt.
I need to hire a teenager to help me with my paperwork...
Let's see... 7/27/2013 ... Giant's game, half-frame
Dektol 1:9 on fresh Tri-X 400 TX 135-36 12/2014 batch 1781
5 minutes 68-degrees F
Aim CI 0.4 - 0.5
Actual CI possibly 0.4 - 0.5 but didn't do sensitometry.
(I deliberately under-developed to aim to a Grade 3 paper instead of Grade 2, long story but I was going for maximum grain).
p.s. The photo is a flat-bed scan of a print. I mention that because so many grain examples are negative scans. Negative scanning can cause exaggeration or misrepresentation of actual grain. This is the grain on a print.
Tmax or tmax rs if dev times are too long for you. I find that stuff hot sometimes, forcing sub 5 min dev times on tri x
ORWO just ran some kind of a promotion so that's likely why it is sold out. On the other hand, the good news is that it is sold out. As long as people keep buying it they hopefully will keep making it! I have plenty of UN-54 right now so I didn't get in on the deal.
Can't say I've used Plus-X recently, if ever, but I have also heard many comment that UN-54 is a good replacement for Plus-X. You can sign up on ORWOs website for promotional emails. If you are concerned about spam, it isn't a problem because they don't send very many.
I don't know the answer to your original question, why developing times are shorter for some films (and why fixing times are shorter for some films). I understand the pleasure of getting used to processing times and procedures, and the reluctance to change for external reasons.
And I hope this discussion thread continues and gives us some answers as to "why" some films need less developing time, because I am curious to know.
But you've got a dilemma to solve, finding a replacement for a favorite film. I wish you luck, and would suggest you look for RESULTS. You'll get accustomed to any new process routine that is required for the replacement.
The journey I took involved quite a few trials and landed me at TMY-2 4x5 to replace 35mm Panatomic-X. So don't be surprised to find your road ahead is disruptive and leads you to an unexpected combination.
I appreciate the "difference" between a tabular grain film and a traditional grain structure, and at 100 speed there are some options for you. I'd recommend choosing between tabular and traditional grain FIRST. Then let other decisions follow.
Hopefully, your stash of Plus-X will last long enough for you to confirm your decision. Keep enough on-hand so you can occasionally check your options against your benchmark. I've found that occasionally revisiting Panatomic-X reassures me of my decision - and it shows me how minor the difference really is.
p.s. I work for Kodak (where What's Next Starts Now) but opinions and positions I take are not necessarily those of EKC.
If it's just looking for an alternative to Plux-X, FP4+, maybe give Acros a try. I assume traditional grain since you are looking for a replacement for Plus-X so I don't include TMX or Delta 100. Acros is somewhat in between IMO, at least in look. Neither of these are Plux-X and will look slightly different, but a shot isn't going to succeed or fail based on being made on Plus-X or FP4+. If one would get the shot nicely, the other will too. Acros is a bit different due to the more significant difference in spectral response and reciprocity failure, the former making the look "more different" and the latter affecting low light.
This doesn't address development time, but just develop for as long as it takes.
I agree the decision for-or-against tabular grain is one of the first decisions to make, because the sharpness difference is noticeable. You can easily identify your feelings about it. I won't try to sway the decision one way or the other because it is a personal choice.