Well, I certainly made some assumptions about your workload that may not be true. What do you use it for?

As bulk storage (backups, media streaming, etc), random 4k reads are usually not going to be the limiting factor, except maybe for the occasional indexing by Plex etc. If you’ve instead got lots of small files you’re accessing, or are hosting something like a busy database/web server on here, then you could see significant boost, but not anywhere near as significant as just co-locating the service and the data. If your workload involves a lot of writes, then I would stay away. The MTTF on “cacheless” SSDs is pretty garbage, which seems like the biggest issue to me.

Also, didn’t mean to suggest buying nicer drives, just using an older one I was familiar with as reference. I recently bought the 2TB 970 evo plus on sale for $80 each, which was in your price range, but not sure if that pricing made it to the UK.

You probably don’t need that kind of read/write performance in your average NAS because you’re almost certainly going to be network limited. Not sure what the specs on these cheap ones are, but something like a Samsung 970 evo from a few years ago would more than saturate a 10g link, so doubling that wouldn’t really help.

That said, I recently built a 4 M.2 drive raid0 on my homelab server for some read heavy workloads, and things scaled close to how you’d expect with just mdadm+ext4 (about 80% of the drives’ theoretical maximum bandwidth in fio test). If you can actually use the extra IOPS or disk bandwidth, it works pretty well and was easy to do.