He could probably run an NFS server that isn’t a closed box, and have that just use the Synology box as storage for that server. That’d give whatever options Linux and/or the NFS server you want to run have for giving fair prioritization to writes, or increasing cache size (like, say he has bursty load and blows through the cache on the Synology NAS, but a Linux NFS server with more write cache available could potentially just slurp up writes quickly and then more-slowly hand them off to the NAS).

Honestly, though, I think that a preferable option, if one doesn’t want to mess with client global VM options (which wouldn’t be my first choice, but it sounds like OP is okay with it) is just to crank up the timeout options on the NFS clients, as I mention in my other comment, if he just doesn’t want timeout errors to percolate up and doesn’t mind the NAS taking a while to finish whatever it’s doing in some situations. It’s possible that he tried that, but I didn’t see it in his post.

NFSv4 has leases, and — I haven’t tested it, but it’s plausible to me from a protocol standpoint — it might be possible that it can be set up such that as long as a lease can be re-acquired, it doesn’t time out outstanding file operations, even if they’re taking a long time. The Synology NAS might be able to avoid timing out on that as long as it’s reachable, even if it’s doing a lot of writing. That’d still let you know if you had your NFS server wedge or lost connectivity to it, because your leases would go away within a bounded amount of time, but might not time out on time to complete other operations. No guarantee, just it’s something that I might go look into if I were hitting this myself.