The difference between Saudi and Qatar world cups will be seen when it actually happens and we can only make assumptions so far. But the biggest difference was that Qatar just didn't had any stadiums or anything, and had to change the whole timing of it, we literally saw world cup in November because of them, makes no sense at all, and it was a stupid decision, hell it was proven to be corrupt and just fired everyone involved, but because of legal reasons (since Qatar already spent a lot) they couldn't take it back, so it was known it was bribery and yet they still had to go with it.
Saudi could be different, in the sense that they at least already have stadiums and infrastructure in place, they could make some improvements, but they are not going to spend billions to actually build it. I feel like it is still not as good as lets say England or whatever, but it is definitely good and I believe it should be better. Not guaranteed though, we still have to wait and see.
But if I recall correctly, no decision has been made about the timing of the Saudi Arabia World Cup and while you are right that Qatar was far less prepared, I think there will be huge infrastructure projects going on in Saudi Arabia. There will at least be modernization going on. But in terms of modernization they have the synergy effects I think from the Olympic Games, winter Olympic Games I should say! That's the frist test Saudi Arabia will go through and if they can prepare for Winter Olympic games, they can build the stadiums right.
But what I found a couple of days ago was how spending on players already dramatically decreased. They were close to a billion in 23/24 and now they are at less than half a billion in summer transfer window 24/5, which means the last transfer window.
I never thought their strategy could be sustainable and if they want their league to keep growing, I think they can't avoid continued high spending on essentially everything that high quality football requires. Managers, coaches, players, stadiums, infrastructure, everything. Now this supposedly still continues in some of the aforementioned areas, but in order to raise the level of their players, they will need to keep importing players from the best leagues.