I highly doubt that he would be getting Saudi citizenship, that is basically a rumour to make Saudi Arabia to look better, just the fact that you can't live with your girlfriend there, and he was given a special consideration, when any other person can't, is the proof that he won't be doing it.
In any case, Beckham deal was basically filled with incentives, whereas Ronaldo straight up gets this much money, they are not the same thing at all and it would not be considered the same now. Plus Beckham didn't get it in a year, he got it over course of many years, Ronaldo is getting a ton of money if we are looking at the whole thing and maybe even after that.
In Ronaldo's case also, he is not getting the entire amount upfront. $250 million will be paid over several installments, during his stay with Al Nassr. And then there is another payment for $250 million (so the total goes to $500 million), which is linked to Ronaldo being a part of the Saudi team, which will bid to host the 2030 World Cup. Usually the bid team includes only citizens of that particular country and that is the reason why there is speculation on Ronaldo accepting citizenship from Saudi Arabia. But we need to remember here that dual citizenship is not recognized in Saudi Arabia and therefore if he gets the Saudi passport, then Ronaldo need to forfeit the Portuguese nationality.