The difference between standard mail-in ballots and absentee ballots is huge. First, mail fraud requires that the real voter never returns their ballot for every fraudulent vote. When a 2nd ballot comes in (the real one, or a replacement if they actually managed to get ahold of originals and the voter requests it because they never received their ballot), the voter is contacted and the ballots are examined to determine what happened. For 100% mail-in systems, there are no absentee ballots. They're irrelevant. Second, large-scale fraud requires massive coordination in order to create voters who don't exist and create counterfeit ballots linked to those voters. It requires access to the voter database and the ballot processing equipment in order to get them into the mail stream without being blatantly obvious. Doing so on any sort of scale would require the system to already be so thoroughly compromised that the fraud would be simply a matter of course at that point.
So no, while the system is vulnerable to certain frauds, wholesale fraud is not one of those vulnerabilities. Being "way more susceptible to fraud" is only obvious if you think about it for a few minutes and then stop thinking before you actually get to the important parts.