Taxis are pretty readily available at FDH and the fare to the port will be about 15EUR.

I would say you have a reasonable chance of getting the boat to Bregenz. Tickets can be bought on board the ship (cash only - €15.80) so as long as you get there by the departure time it's fine.

If you do miss that one, you still have the option of the ferry to Romanshorn (hourly until 20:27). The Lindau - Rorschach Option probably won't work, as the last departure is 19:05 (19:30 from Wasserburg to Rorschach).

The rail replacement buses are a bit of a PITA to be honest. From FDH, you would need to get to Lindau, then take a bus to Lochau-Hörbranz, then get back on the train to Bregenz or Feldkirch. I live in Bregenz and work in Lindau and have been driving in every day since the trains have been out.

I would aim for the 18:25 boat to Bregenz, with Romanshorn as a back-up option if you miss it.