I use forms as well. It’s the only thing I found that really works.
I have tried to use what are sometimes called breast enhancers. These are really backless bras that look like chicken fillets, are curved and sticky on the inside, and clip together. They are meant for women in backless gowns and it holds the breasts together. It is supposed to work on men’s breasts too, but I found it is easy to expand the chest just a bit and the sticky side just pops off.
I know I can cup my male breasts and push them up and together to make it appear I have cleavage. But I have yet been able to do the same thing using tape. I push up one where I want it and tape the skin. But it just sags as soon as I let go.