Yeah, trying to spot a RPi 4 in stock – and selling at MSRP – has been rarer than people spotting the Loch Ness Monster. 
The RPi 3 B has enough horsepower for Syncthing with a little work tweaking the setup.
Although slow, it might seem fast compared to the Ethernet interface because it’s 100BASE-TX on the RPi 3 (my earlier post about the RPi 3 B+ might be of interest: Too slow for school - #5 by gadget).
By default the database files are located near Syncthing’s config file. On Linux it’s usually under ~/.config/syncthing/ in a subdirectory like index-v0.14.0.d for the user running Syncthing.
It looks like your 10TB WD Elements (WDBWLG0100HBK-NESN) is CMR rather than SMR, which is good because SMR + lots of small files + lots of disk writes often doesn’t work well (especially when combined with NTFS).
Much better… 
A creative solution that’d work, but a few other options that might be of interest… 
- Linux Reader (https://diskinternals.com/linux-reader/)
- explore2fs (https://explore2fspe.sourceforge.io/)
- Ext2 IFS For Windows (http://fs-driver.org/)