Eliseo, it's nice to hear of someone else making up their own propagation media, it can be frustrating, but it's rewarding to solve a problem. It is also a good guess that commercial growers of Pk and it's hybrids have the answer to your problem.
Unlike you, I have no experience of phrag propagation, but I've been thinking about your problem and trying to figure out what I modifications I would try, and which I'd try first.
If I've understood your posts correctly, you have a medium based on P668, on which most of your phrags grow well, except Pk and it's hybrids. A reduced NH4 version of this supports Pk, but frequent replating is needed, in part due to rapid acidification. The rapid acidification suggests that the NH4 is taken up quite quickly and possibly depleted. You have a NO3 only medium based on MSU with banana puree that supports growth for longer but the growth on this is slower. One possibility is that phrags prefer a reduced form of N under in vitro conditions, just that some species are sensitive to the acidification caused by NH4 use. The only way to find out if this is the main issue, is to try a modification that gets around this problem.
There are two approaches that could work, the first is to replace NH4 with glutamine, either in a P668-like recipe or in your MSU formulation. It's a good guess that phrags will use glutamine. Glutamine is anionic so not acidifying, has two N atoms per molecule and feeds straight into a primary N assimilation pathway and it is the main source of N in the BM-1 medium that has been used for phrag propagation. The second approach that I favour much less, is to stick with the NH4 but add it as salts of organic acids (OA's) from the citric acid cycle (but I'll leave this alone for now as there are some complications with this). I think it could be worth your while to have a look at BM-1 just for ideas, rather than as a recommendation to use it as it is.
BM-1 is a totally different type of medium to P668. You can see the recipe here:
https://dephyte.com/BM-1-Terrestrial-Orchid-Medium , just click the specification tab. This medium is recommended by this company for paphs and phrags. It has very low salt, no NO3 and no NH4. All the N comes from a combination of glutamine and casein hydrolysate. What is common between P668 and BM-1, well not a lot, they are almost totally different types of medium. They do both have a supply of reduced N, so NH4 in P668 and amino acids in BM-1. The issue I would have with this medium is no added Ca, so maybe this is a contaminant in the casein hydrolysate? It may seem strange to add glutamine when you are already adding peptone or protein hydrolysates. The answer is that protein hydrolysates can differ in their ability to support growth. Some amino acids, e.g. threonine, if too abundant in the mix can be slightly inhibitory to growth, so the profile or balance of different amino acids can be important. In BM-1 they are adding casein hydrolysate at 500 mg/l and glutamine at 100 mg/l , and hydrolysates also contain oligopeptides that are not all available, which means only a fraction of the 500 mg/l is as free amino acids by weight, and so glutamine will be by far the most abundant amino acid in this medium. On a practical level I note that people who use glutamine tend to filter sterilise it and add after autoclaving. I think this is because it can partially break down into glutamic acid and NH4 during autoclaving, however BM-1 is sold as complete medium in which case autoclaving it would be the only option, so I'm not sure how important this really is, but it's good to be aware of this issue.
Another issue is the frequency of replating needed on the media that give the best growth. It could be that you are working to a osmolality limit. The P668 medium might be near the osmolality optimum and your MSU medium above the optimum. If this is the case it might be possible to make adjustments in your recipes that get you to a better compromise between rapid growth and frequency of replating. I have developed a script that estimates the osmolality of media, it's only rough, but serves as a guide during formulation of media. For example the banana puree will contain some organic acids, sugars, salts, etc., and will add to the osmolality, so you might be able to reduce the sucrose a fraction if you're adding this supplement in order to offset the osmolality increase. So essentially if a compound added isn't used effectively as a nutrient, is present in excess or doesn't serve an essential function (like MES is needed as a buffer), then replacing it with something that is, would move your recipe towards a better balance between growth rate and replating frequency. Another example; P668 has a lot of chloride (3mM) ,and plants need only a tiny fraction of this, it's place can be taken by something that is more useful. If all the NO3 isn't being used efficiently then reducing it from 19.7 mM to 10 mM say, gives you another 9.7 mM to play with. We can discuss this more if you wish.
