Hello Meran, I see your plight, wish you were available for a shopping trip as I am sure we could fit you into something that looks good. It seems to me you have brought up a super important point, breast forms are only as good as your bra, or the other way around if you look at it. Every girl is different has a different body shape, muscle structure, etc. Here are some basic steps:
Find you shape according to fashion and body type normals. The fashion industry has been fiddling with and narrowing down the perfect visual appealing female shape for hundreds of year, it will never be done of course as preferences change as generations change. However I found Kibbe’s method the best for categories of the 2020’s. Go watch a video, I like Aly Art, on the topic and determine you shape. Most of us MtF’s are more on the dramatic side of things due to our birth, but not always and lots of times we are typing to erase that birth dramatic-ism, best to work with it a bit vs fight it.
Your breasts are part of your shape and anything outside of your type will look weird/unnatural. So time to go bra shopping, try on a few, try on a lot, get the department store girls (if they still do this) or Victoria Secret girls to measure you. Decide on perkiness, roundness, position, etc. Remember all bras are not made the same. As for the forms, you have to do the same for them as your bra, but basic tear drops can be positioned lots of ways and I found them the most universal. Generally you are going to need a full coverage bra to hold them in, unless you have the glue, but that can be a real mess. Meran as you also point out once you have a bit up front because of HRT or whatever the full forms no longer look right. You might need to try a booster form or half form. What I found for a more soft look is a sport bra with no under wire, they hold everything in but soften the edges, only issue is they are right where you put them not a lot of movement, plus they have no hang. For more bounce I use a cotton full coverage underwire with an old school tear drop silicone.
Anything other the silicone tends to not move right and shape well with a bra. Foam, socks, bird seed, play doh (sry), cotton, etc just never moves correctly with your natural body. I found the forms with the extra sides/flaps for under the arm pits is too much for us MtF’s as we have a natural amount of side muscle there and it brings the mass to far over to the outside. Vanishing edges are good for glue on applications and topless photos and going braless but have a much greater sweat factor so not an all day type of thing, a few hours max. My guess the plates and body suits are the same.
Hope this helps.