Need a better NAS for running syncthing (2020)

A SPK to install is no problem. I think, its a “no-arch” package, so you can use on each type of Synology NAS.

Your idea is at least justified, a “DS115j” is a bit small. Nevertheless, there are tuning options in Syncthing for this, these are in the documentation.

If you want to invest, a DS218 would be the minimum for this task, better would be a DS218+ which could still be expanded in RAM, officially up to 6 GB, unofficially there is much more possible.

The problem with a Synology is that you do not have a “pure” Linux server, since the RAM is still loaded with Synology specific processes, such as the DSM, the APPs running on it, etc. If you want to work with the above scope, a good portion of the 256 MB RAM will already be gone.