IMHO we should be utilising the Chiltern service to relieve the WCML, not push passengers onto it, and you do that by improving it so not only Aylesbury passengers prefer Aylesbury over Tring, but those midway with a choice head for Aylesbury rather than Tring as well.
There isn't a capacity problem on the Tring semifasts. And any problem that may exist on specific trains would be solved by extending them from the usual 8 to 12 coaches.
The issue for Aylesbury is that while it is nice and civilised there's not much scope to make it much quicker due to the low speeds permitted on London Underground's infrastructure (though it's noticeably quicker north thereof). Thus for people on that side of Aylesbury, a drive to Tring (as they'd drive to Aylesbury or Vale Parkway anyway) and a 35 minute ish train in is always going to win.
I suspect there will however be some small growth as Aylesbury Vale Parkway becomes more of an Aylesbury North, with the massive growth of housing (some of it quite high density) surrounding the station. When built it was in the middle of a field, now it's half so and on the other side suburban.
Cut the Met back to a relocated Ricky (short length of four tracking needed, as originally intended) and/or Watford, lengthen the platforms to 8x20m, upgrade the tracks for speeds comparable to the WCML's AC lines, and perhaps, even install OHLE as the tracks would now be segregated. I suspect that might still work out cheaper than trying to squeeze ever more inner suburban services onto the very congested (and already 12x20m) WCML.
Post COVID the south WCML isn't, passenger-wise, totally full. Most services are 8 car and coping, extending the lot to 12* would provide all the capacity needed for the foreseeable.
* I'm aware of the challenges posed by the two 8-car platforms 9 and 10, but you could probably get to most services at 240m and I suspect we will with the coming of the 730s, as 120m will be grossly insufficient for the peaks.