NAS.
Over the last 24 months I've built 300TB (a mix of 10 and 14TB disks) for $2500 in disks. I could do that right now for $2100. A 18TB LTO9 tape is more expensive than what I'm paying per TB for 14TB disks.
$700 in hardware to build the NAS with 25 bays.
Glacier would cost you $1080/mo in storage fees alone (300,000GB @ $0.0036) not including the $0.09/GB to get any data back out. Deep Glacier is less (by half, for storage), but comes with strings attached.
Don't forget to factor in labor hours of what it's going to cost you to maintain a tape library or a local server in general.
Are you charging clients for long term storage after a project is complete? If not, you should be.
Compressing SD DVD's to 265 is a terrible suggestion. 265 should only be used for high def content, ideally 4K media.