https://www.theatlantic.com/health/archive/2021/10/mix-match-booster-covid-vaccine/620395/TL:DR/freak you paywall version:
-Very limited study of only 50 people, not tested nearly enough to be a deciding factor.
-Two J&J shots produced lowest antibody count
-J&J+Moderna slightly better than J&J+Pfizer
-Moderna regimen with Moderna booster produced highest amount overall.
-Pfizer regimen with Moderna booster produced second most
-Moderna regimen with Pfizer booster produced the third most.
Again, not enough of a sample size to trust it but it looks like mixing and matching will produce better results for people who did not receive the Moderna regimen.