I did end up ditching the modem since I wrote the article. I ended up using a TP-Link 8411 router though. Having everything TP-Link has its benefits for observability and maintenance.
That's still only 1/3rd of a single U.2 or a 6th or single U.3 drive... and the IOPS over SMB/NFS is significantly lower than a local drive, even with a big ethernet pipe.