I cannot recommend any USB-connected drive for long-term use. (Only for portable devices that get plugged in for a little while at a time.) In the long term, any USB drive will randomly reset during periods of heavy use -- including heavy writes, meaning some data will get lost.

USB enclosures tend to just crap out completely after a year or two, if used continuously on a server. I know because I twice used 1TB external drives with OpenWRT (home router) devices. The data will be safe on the drive, but you'll have to replace the enclosure.

1. My first recommendation would be to look very carefully at the chassis and see if there's any way at all to fit another SSD inside it. 2.5" SSD's are usually thinner than 2.5" hard drives, so it may be possible, and most motherboards have more SATA ports than they need.

Is there possibly an NVMe slot on the motherboard? Or an open PCIe slot where you could put an NVMe adapter?

2. My second recommendation would be using a 2.5" hard drive. Newegg has a 5TB one for $135, but unfortunately that's as large as they seem to go. It will be a bit slower than an SSD, but still probably around 150MB/s for sequential access.

3. My third recommendation, if money is really tight, would be an additional server, with a large 3.5" hard drive. This will be a lot cheaper than an 8TB SSD, but adds complexity, electricity use, space use, and possibly fan noise.