Yeah the first Gigantor was about 2006 and the last about 10 years ago, they were a series of smaller tours. He's also done the Megacruises which are pretty much the same but on a smaller touring scale. So he does know how it all works and I imagine also knows the risks and the costs, but major Justis doesn't.
Justis was only recently (in the last few years) promoted to the top end of the scale at Megadeth Inc (or whatever it's called). From memory around about 2017 he was CEO of the wines and winery part of the business and then in the last year or two Dave posted a congratulations on taking over the band business. However I did also find an article from 2016 where Justis actually talks about dropping Havok from the tour and his title was head of Mustaine Management. It's only a guess that Justis has worked his way through several facets of the business, i.e Mustaine Managenment, Mustaine Wines, etc and now he's head of the Mustaine corporation of companies. In Australia I know how to search business and company names for ownership and management details but I don't know how to do that for the US so I'm only assuming those details based on how most family trust type businesses here work.
A tour with Kreator and and Testament would be a monster tour I can't see Dave wanting to do that because as good as Megadeth might be both Kreator and Testament have proven recently that they are much better live and Dave wouldn't want to play third fiddle on a tour he organises. Enforced and AA are probably a better chance to make a three band tour with Megadeth, they might still blow Dave off the stage but they have less fans so the majority of the audience will still be there to see Dave.