In your situation, you use the same account for PC and Xbox.
This means that you have access to your Characters on either platform, but also requires that you buy each version of Dawntrail to have access to it on both platforms for example.

So, if you buy Dawntrail on Xbox only, you can't access it while on your PC, you can still play on PC, but would need to buy the expansion on each platform to access its content (just an example).

This means that you only pay one subscription to access both platforms.

Except but this, the Xbox version has a secondary requirement to play this game, you need to have an active Game Pass (Core or Ultimate) subscription as well.
So, this means that you have to pay for Game Pass and also this games subscription to be able to play on Xbox FFXIV.

You can pay for your Xbox subscription on the MogStation since you have the PC version to get around the FFXIV coin thing, which I believe is only for Xbox only subscribers.
But you must also pay for Game Pass to gain access to your Xbox version.

One subscription on MogStation = PC access
Game Pass on Xbox = PC and Xbox access