Having been someone who played in those events you listed above I can tell you that roughly 99.7 percent of the matches now are 100 percent legit. And of the less than 1 percent that have one guy getting paid off, it is almost only for a couple games or maybe a set, yes not even the entire match.
Back when I played I'd say the number was around 95 percent, but now with the Tennis Integrity Unit involved along with a ton of Sportsbooks reporting in when they see a match that might be fixed you have to be a complete idiot to throw a match. It is almost impossible not to get caught. And on top of that every player knows the phone number to call if you are approached and/or take a bribe. You have so many "outs" even if you are "in deep" you'll get a pass if you just report what is going on.
In other words it isn't worth it to take a couple thousand bucks (there was a mob out of Bulgaria that had 5 players on payroll and even they were not getting big bribes) to throw a couple games. The ITF (International Tennis Federation) are now handing out lifetime bans if you get caught (remember you have multiple chances to report yourself) so you have to be a complete fucking moron to take a bribe nowadays. Does that mean the sport is 100 percent clean, of course not. But I'll say again, you are an absolute moron if somebody approaches you at a tournament (I know of 4 former players where this happened multiple times) asking if you want to make "some easy money". It is so not worth it. You will get caught, you ain't going to get away with it, and the risk is so not worth it.
Pro tennis now is actually one of the most fair sports you can bet on. The odds (pardon the pun) where a player "throws a match" because he was bribed is infinitesimal.
Oh and for the record for all the men's players that have been caught in the past 5 years, almost none have taken place at a Challenger Event, they are all at "Futures" Events. And of those, almost all of those were in Eastern Europe (on clay).
The "on clay" part was for Smitty 😆