What is the minimum amount of scapular work one needs, in order to maintain good posture when doing extremely high pressing volume, considering one starts with a back strength on elite level and a pressing strength which is sub-par?
The rule of thumb is to balance pressing work with pulling volume (so, 1:1), or if working to correct a postural issue, then double the pulling volume (1:2).
But this is just a guide, and we have to remember that movement volume, like calories, is not actually a fungible quantity. We can aim for certain easily-tracked benchmarks (1 push-up :: 1 ring row) but we must remember that the internal mechanisms are much more complex, in fact too complex to reasonably track. ("I did ring rows at an angle such that I bore 70% of bodyweight but my push-ups bear 75% of my weight, and some stabilizer muscles in the push-up are prime movers in the pull-up; my lats also got some work from deadlifting the other day, but that should be OK because my deadlift is way stronger than my pull-ups...")
This is especially true for gymnastic movements, which don't lend themselves to quantification. It's less true for overhead work, since that usually involves enough scapular retraction that push/pull imbalances in the shoulder aren't so pronounced compared to horizontal pushes like bench press or push-ups.
If starting with extremely strong pulling muscles (in the relevant plane), maybe some rearward bias in the shoulders, and a deficit of pressing strength, I could see myself being okay with a 2:1 push/pull ratio to get my pressing caught up. Lower than that in the horizontal plane would make me think twice. During a training cycle where I want to de-emphasize but mostly maintain pulling strength while focusing on pushing strength, I would pick low-load accessory exercises for pulling, just to keep everything steady. For instance, I might use two finishers, alternating: in one workout do a couple sets of unloaded pull-ups, in the other, a couple loaded sets where the load is high but I stay in a comfortable rep range.