I mean; the NAS would have some built in redundancy via RAID 5 or 6 or whatever, but you wouldn’t have an offsite backup. What you’d wanna look into is something like Backblaze B2, but even that is going to be $1800 a month, so at that point I would say build a 2nd NAS and pay for it to be in a data center, that would only be a couple hundred a year, or even just run it at your house and run a nightly backup.
Also idk where OP is finding LTO 9 for $20k, HP makes a LTO 9 drive for $4500
How often are you actually needing to access the 300TB? If 250TB are “cold storage”, then LTO is the way (you can rent the readers usually, rather than buy)
If you’re needing to have access but not edit from, NAS is the way, 300TB wouldn’t even be THAT expensive (still expensive), just slow to move to, but once you’re up and running a decent rig should last years.
If you’re needing to access all 300TB, then you’re looking at a LTT style NAS that needs to handle read and write from multiple users at a time, and that’s gonna be the real $$$.
I feel like you might do well from a mixture of all of these. A smallish NAS for day to day/project use, and once that project is done you move it to the big “slow” server for onsite backup, and once every 2-3mo you rent the LTO drive and load up a few tapes, and ship them off to the void for offsite backup and cold storage.