You seem to be conflating the use of a VPN server w/ port forwarding. When you configure a VPN server on the router, you do NOT port forward anything. Instead, you get connected to the router's VPN server, which then gives you access to the LAN, which includes the NAS if it's a LAN device. Of course, Merlin doesn't support WG (WireGuard) as either client or server at this time.
Also, your example of "NAS >>GL-MT1300 [running wireguard vpn server] >> AX58U Router" suggests *outbound* access via a VPN, but that requires a VPN *client*, NOT a server.
If you want remote access to the NAS which is secured by a WG server, then I suppose you could establish the WG server on the GL-MT1300, then port forward from the WAN of the primary router to the service port of the WG server, which in turn provides access to the rest of the LAN (including the NAS). Just so long as the GL-MT1300 allows access to the WG server from is *LAN* side. Since I don't use that router, I can't say for sure, but many times oem's will limit access over the device's WAN. If that's the case, you'd have to place the NAS behind the GL-MT1300's WAN, on its own IP network. But that then complicates local access by requiring *local* port forwarding between your private network and the IP network of the GL-MT1300.
That's why in the long run, it may just be simpler to stick w/ the OpenVPN server on the router, despite the lesser performance. Frankly, I find while on the road that the amount of bandwidth available is so limited anyway, it doesn't generally make all that much difference. IOW, if your typical hotel or wifi cafe is only offering say 15Mbps, the choice of OpenVPN or WG just doesn't matter all that much.