I'm not singularly worried about mail in voting. I do think we could (should) improve voter security and should be able to do so pretty easily. it's shitty as is. and I think there are other definite issues with voting, noted above that are perhaps more directly malicious in trying to swing votes than the possibility of misusing mail-in votes. I also dont expect fraud to play a major role in elections. Like, who is out there defrauding elections on a meaningful scale? The established ways to influence elections legally are already widespread, gerrymandering and subtle voter suppressions, or "ballot security" voter intimidation (now legal).
But contrast those big issues specifically designed to sway elections on a coordinated and large scale, I dont see it for mail-ins.
Even those getting dead peoples ballots or whatever, I dont know anybody who'd even use those, and if they did i dont think it'd make a measurable impact when you really think about it.
Like, how does the mail in voter fraud scam work. your specific WA example is a decent option but I dont think you can pull that off unless it's very close and poorly run election. Otherwise you'd need a pretty major coordinated fraudulent scam that i'm just not seeing as likely or really even feasible.