I'm sorry but I'm a total novice at these things. Can you provide the directions to do that, or a link to some please?
ETA: Here's my setup (as I've envisioned it, but maybe not final depending on the advice I receive here):
OpenWRT router LAN 1 > ISP gateway LAN 1 in cascade.
Pi > ISP gateway LAN 2.
NAS > ISP gateway LAN 3 (with only transmission routed through VPN on OpenWRT router).
OR
NAS > OpenWRT router LAN 2 (still with only transmission traffic routed through VPN and other NAS traffic bypassing VPN).
All other devices > ISP gateway LAN or WiFi, but with the option of connecting to OpenWRT WiFi (meaning different SSID from ISP gateway) so that they can go through VPN.
If something like that is not possible, then I can setup IP passthrough on ISP gateway and basically use it as just a modem with my entire network running on the OpenWRT router.
EDIT 2: I'm guessing something like this is what I need. In particular, the second bullet point under "IPv4/IPv6/Port-Based Policies": VPN Policy-Based Routing + Web UI -- Discussion
EDIT 3: There's also this: https://openwrt.org/docs/guide-user/network/wifi/relay_configuration