I'm running a lot closer to the edge with my 97% on the volume - I'm doing cleanup & organization in the background, but yea, there comes a point when you have to stop. There are huge advantages with a new NAS - mostly in that you'll not touch it for that much longer. Doing SHR2 (RAID6, but with extra features) means you lose 2 drives, and doing that in a 5 bay enclosure means you only have the capacity of 3 drives. Stepping up to a 6/8 bay enclosure means you're getting 4-6 drives of usable space.
My biggest complaint about the current Synology lineup is 4x 1gb ports is bunk in my opinion for 2019 'new products'. Synology doesn't have a combo 10gbps ethernet + 2x SSD caching card like QNAP does, so in their single slot models we have to choose one. I just did up a proposal for a RS3618xs unit for a client & we're filling it with 12x 10TB drives due to the cost per gig. Doing an 8 bay unit with 10TB drives would get you like 54TB usable. Stepping up to 12TB drives would only gain you 6(ish)TB, or 14TB would gain 12(ish)TB. IMHO the price jumps don't make up for the minimal gains in capacity.
Just playing with price & options:
DS2419+ $1,500 (12bay)
8TB ext drive (WD/Seagate) 5400rpm (shuck the drive out of the enclosure) $140 each
$3,180 for ~74TB of usable
DS1819+ - $905 (8bay)
10TB drives (shuck'ed) $200 each
10TB NAS drives $270 each (WD RED, 5400rpm)
$2,505 for ~54TB usable space with shucked drives
$3,065 with 'NAS' drives
Performance of the three units above would be about equal - more spindles, on slightly slower drives. Both setups could take a 10gbps card or a dual SSD cache card. As for shuck'ing external drives, just make sure the model you're getting has their 'compute' brand disks inside, not the Archival one. With any of the arrays having a space onsite really makes it easier to deal with any possible failure, but drives don't die like they use to (touch/knock on wood).