I think his opposition ranged from bad to fair. Comparing all the different heavyweight eras, I thought his was the worst.
My thinking is that you might get lucky with a weak era if you're only around the top for a few years. For example I think it's reasonable to say that Marciano ruled over an era in transition when the best around (Charles, Walcott and Louis) all had passed their best by different degrees. But Louis beat contenders for over 15 years and faced all kind of styles but still ruled supreme for most of that time.
Your thinking is spot on. He was pumping contenders from 1935 until he lost his title in 1950. He was champ and defending from 1937-1950. To think that some good fighters didn't crop up during that time is fanciful at best. Tyson lasted about 4 years fighting the best. Holmes was 5 or excusing the late reign shenanigans, let's say 7. Frazier you might give 5 years, Lewis 11 with two losses in that. Marciano maybe 4'ish, Ali 14 with some losses. It's little wonder Ali and Louis are default #1 and #2 to most. It's insane how long they were able to be or re-reach #1 for. Louis did his years without loss whereas Ali obviously fought tougher top end opponents but was checked along the journey. I can't believe how easily some fob Joe's achievements off. The man was insane. You're not going 15 years without some stern tests. Super long dominance can/will also check or upend the careers of others that would have made bigger reputations for themselves otherwise.
Hard to say. IMO his best wins are over Schmeling (ATG win), Walcott (ATG win), Conn (great win, Conn's a bit small so I wouldn't say ATG win), John Henry Lewis (ATG LHW but notably unsuccessful HW), Max Baer (should be a great win but Max broke his right hand prior to the bout, so he was pretty much helpless), Primo Carnera (past prime), Jack Sharkey (washed). He has other decent wins like Farr and Pastor, but these were the standout. I'd say he's a BIT overrated, mainly due to the nut hugging some people tend to do. Overall though, still my number 2 HW and top 25 all time P4P.