
The creator didn't have much imagination and cut the series rather short when there was tons of room to have Yamcha grow.
Going on Namek and training with nails was a good start but I feel he really undersold just how much gains Yamcha would have gotten from training with someone with a PL of 40k and having his PL unlocked.
Training in the gravity room would have been the next best place to keep up relevancy. Basically, Yamcha could have trained alongside Goku when he was heading to Namek and likely obtain a similar power level to him. This would have allowed him to help out in some capacity on Namek.
From frieza arc to cell saga, his gains would have stagnated a bit but could have still obtain relevancy by basically keeping up with gravity training and possibly learning kai-o-ken technique from Goku. During the 7 year gap between cell saga to the android saga he could have transformed himself into an android and basically reach SSJ3+ tier to come back into series as possible deterrent to Majin Buu.
From end of Buu arc to start of daimo, Yamcha-reborn could have then had decent good fights handling many of the threats that Son Goku and Vegeta faced in series. By Battle-of-Gods, Yamcha reborn would have been at least SSG+ tier to put a decent fight against Beerus. Yamcha could have then taken the opportunity to train with Whis at Beerus palace. By U6 vs U7 match, he would have been a decent candidate in place of piccolo to represent their universe and likely be a much stronger candidate than Golden Frieza for ToP