You can try Syncthing. It sync folders peer-to-peer when at least two devices are online. If you already have a NAS, you could install Syncthing there too to ensure that at least one device is always online.
I just got it set up between my Desktop and Android device. I can’t use the drive directly as a cloud (but I don’t want to since I don’t expose my NAS to the internet), so I am using the built in local sync. I sync at home via WAN on my WiFi only. Here are the steps that worked for me.
Check your DSM and Synology Drive Client versions. It was a big headache before I figured this out. Without an up to date version you won’t even see the Backup and Sync menu (but the App tells you nothing). Synology Docs for Android
Desktop: Create the LogSeq Graph on My Drive in Synology Drive via Desktop
Desktop: Right click the LogSeq folder and select SynologyDrive - Pin Local Copy Permanently to make it accessible offline. (Optional)
Android (Synology Drive App): In Files, click the three dots to the right of your LogSeq folder and select Enable Sync Task
Android: Go to More (bottom menu) - Backup and Sync Tasks - Sync Tasks
Android: Click the three dots to the right of your LogSeq folder and select Edit Task
Android: Set a local path where you want the LogSeq folder to reside in your Android file system. (e.g. Internal storage/Documents/LogSeq)
Android: Open LogSeq and set up your new graph, using the local folder from step 6.
This will provide a synced solution on as many devices as you like, just be careful about using multiple devices at the same time. You may need to play around with the sync settings (I am keeping the latest modified versions) and frequency, or manually initiate a sync before switching devices.
I haven’t been testing this solution for long. It seems that Android may require a refresh (but happily not reindex) when a change is pushed from Desktop. I think the sync to the local folder is actually the bottleneck here and I may need more testing on the sync frequency side. Desktop seems to refresh automatically.
I started with a Graph on my Android device before I set this up. I exported it as JSON, started my new Graph on desktop, then imported said JSON between steps 1 and 2 to get it off to a good start from the Desktop side.
Probably too late for OP, but hopefully it helps someone else.