Am I parsing it right, that you want 24TB of storage? You could do 3x12TB or 4x8TB. Aim for CMR drives, not SMR; SMR is great at intermittent usage, but not certain parts of NAS, like filling a new drive. Try to get different brands, models, or at least batches; there have been firmware bugs hitting at a certain uptime.
You can get used drives; test them and verify their capacity. I used https://serverpartdeals.com , and they replaced one that didn't work well; they have 12TB drives for $97 right now. I didn't verify they're CMR but I'd be surprised.
Amazon and Newegg have a Synology 4-bay NAS for $400 or less. Mine has been easy to administrate locally. You might work from Synology's hard drive compatibility lists.
A rough total for those is about $700.