I studied Kenpo for a number of years and even though I have been a body builder for the last 4 years, I think I know what direction you're going.
Building mass is a tricky thing because it's pretty much 90% genetics, that is, unless a person is tilting the scales by using illegitimate substances to increase synthesis and metabolic rates.
Judging from the pic, I really do not think you have a whole lot to worry about regarding mass because you already look like you lean more toward that of a Kenyan runner rather than someone shooting for hypertrophy. Believe me, that's no slam. Kenyan runners are fast, heavy on endurance and extremely agile but that's what they are genetically built to do. That said, they still have to practice and work to make the genetics applicable.
As for myself, I'm nearly 70 and wish to compete in the amateur seniors bodybuilding competition in my state and win it. Why? Because most research says that a man my age can only maintain muscle but not build it. Wrong! I'm living proof that the research is incorrect but a person has to be willing to go the distance to get whatever it is that they wish to have no matter what the age.
There is no such thing as overnight champions and let's face it; if it was easy, everyone would be doing it but it's hard work and it takes a lot of dedication and very few people have what it takes even if the genetics were perfect.
Even the OMAD diet takes some dedication but when a person wants to improve the way they feel and look, they simply have to knuckle down and do it. Nothing is easy at first especially if we've been trained our whole lives to live a certain way but what if that "certain way" isn't really all that great? People need to learn to break the mold and go about their lives being terrific instead of just okay. I hate being average. I like being wayyyy above average so, I work at it.
Martial Arts takes practice and dedication, gaining muscle mass and body sculpting takes the same but to be sure, even going through daily life doing whatever demands a good diet and some proper exercise to look as good as we do, right?