It doesn't matter--from the point of view whether their action is legal, that is--provided the warrant was obtained legally and based upon reasonable grounds.
Trump is under criminal investigation in multiple jurisdictions. That's not news. If none of these have turned up sufficient cause for this action,
a) I'll be extremely surprised but I'm biased and feel certain he's guilty of many crimes, small and large.
b) It may be necessary to concede the possibility he is being unfairly subjected to harassment by law enforcement.
c) Bias is obviated by b, so like you say, they'd be nuckin futs to raid a Trump property without really heavy duty evidence and it would be not just surprising but a completely mind blowing bombshell.
d) Regardless, my guess is the 2 years of text messages by Alex Jones which his lawyer sent to the prosecution in the defamation case produced something actionable, emphasis on the word guess, since, like I said, there are several different possible reasons for the raid, sadly.
e) Assuming they haven't lost their minds and decided to hand the next two elections to Republicans in a single day, this is a huge moment in the quest to see that criminals get what's coming to them.