Sounds like a great setup! I haven't seen any other posts in this thread regarding Thunderbolt and I don't have that Flex IO port in my system. I would recommend that you search for other Thunderbolt threads (like
this one) and post your questions there. In my opinion, as long as you have a supported Thunderbolt chipset, your solution won't be specific to the G5 DM, so you should be able to seek advice from others who have working Thunderbolt in different systems.
BTW - it's great to know that the Thunderbolt option exists in our EliteDesk Minis. One more expansion option that makes these little PCs such great hacks.