This is just a suggestion but after the MC loses to a no name he fights in a few underground fights for money like you said but instead of him he finds a pupil to raise to the top in the process he himself begins to grow stronger
The MC is still young so I don't think he'll take on a pupil but he will meet someone in the underground fights who inspires him to keep trying to reach his dream.
It really sounds straight forward. Main.character has a goal, character finds a way to get there. First off he's already a good fighter apparently if he went to fight a significant group with a title, and he gives up when he fights someone who barely picked it up within that year.
But so far I'm not seeing much to keep me reading. You mention.interesting personalities but which one in particular? Also his personality, how will it develop in this story?
A lot of sports anime have started out with an MC who's already good at the sport but I'm trying to do something unique by having my MC begin without a clear dream or goal. His development will involve how he decides to pursue his dream again and how he is able to recover from the humiliating defeat he had when he was in high school.
I haven't thought about it completely but these are some of the characters who will inspire/support the MC:
-A female character who keeps trying her best despite there being no money or fame for female MMA athletes
-A male character who keeps trying his best despite not having enough money for coaching/lessons
-A male character who keeps trying his best despite being a weakling and getting picked on all the time
-A female character who supports/comforts the MC (kind of like Hinata from Naruto but not as shy and quiet)