The €1900 X-H1 is a small handheld camera so it's hardly going to compete with an €10,000 FS7 on the options list.
I use an FW700, (still) a great 4K camera, with a 7Q attached. As most things running a modular set up allows you to solve any problems including ITC between cameras. No reason you can run something like 7Q on the X-H1. So really sound syncing will be fine with this.
The X-H1 only records 14Bit files, the GFX also is 14Bit raw or 8 Bit TIFF internal.
The 7q can do continuous ltc input? That solves that problem then. Great to hear.