@TheMadHatter
Welcome to the AH. Lots of opinions here.
Your setup looks visually appealing but I can see where there would be audio shortcomings.
One aspect of HT and audio systems is the Wife Acceptance Factor (the WAF) and it is a non-audio problem that complicates a lot of home setups. What will the WAF do in this case if major expenses are recommended/desired? What would the WAF say about floor standing speakers and wires?
Your setup solves a lot of problems with your room layout (like all those windows and open areas but produces others. You are correct, for "best" sound out of movies etc for dialogue, you would ideally want your center channel speaker (and L and R) driving right at ear level. Center channel dialogue is one of the chief beefs about many a home theater setup.
I think before investing anything, you should make a list of what you want to fix. What about your existing system isn't up to your standards? You won't know if you've fixed anything if you don't have specific shortcomings to address. It would also be handy to know the Sound Pressure Level (SPL) at the listening chair position. A cheap DB meter from the old Radio Shack does the job just fine. Are you getting enough sound during your use of the system to meet your ears needs? That DB meter will also help tell you very quickly whether you are under powered or not.
The second thing to consider is budget. Do you have a budget? Audio can get really pricey when you have 10 or 15 guys on the AH making recommendations.
I will shut up now and let others chime in. This stuff can be incredibly fun to do. Expensive but fun.
You have a great looking room and setup as it is. That usually means $$ to make it better