You could try maca root, 1500 mg a day. It’s cheap, doesn’t seem to have significant side effects, and some guys swear by it for increasing ejaculate volume. There’s even a study:
I don’t know what the mechanism is, but the study suggests that the volume increase isn’t related to serum levels of T or related hormones, so I’m thinking that it could work just as well for those on TRT.
I also agree that TRT may cause “lower” volume of ejaculate, but it is due to the hypothalamus not being stimulated. If the hpta doesn’t get the “needed” signal from the brain, there will not be any sperm production due to lack of LH and FSH that stimulates the testes to produce both semen and testosterone. In my personal experience I take both T shots and HCG and have plenty of seminal volume and muscle contractions during sex. To be honest my orgasms are stronger and more satisfying than when I was in my 20’s.