Addressing your questions -
1. "Can you up the population max for Metropolis planets?" Consider it done. Though in R4 (which I'm doing final testing there will be a number of new planets) the planets balance will be shifted again.
2. "Not sure if you are willing to do this, but an expanded tech tree would be great". Haven't played the mod but assume that it includes expanding the research tree of the original races too. Two points ... I don't what to change the original races. Secondly the AI does not know how to optimally use the new research as a result to give the human player an unfair advantage. That being said I'll consider adding additional research in the expansion mod where I having been putting things that some consider questionable.
3. "In the map designer, I noticed that several the suns show up as "string not found""
Copy the galaxyscenariodef file from the Maelstrom mod into the galaxy forge folder overwriting the original.
4. "Did you know that you can max out the capital ship allotment but still be able to build dreadnaughts? Can this be changed?" This is a fundamental problem of including a massive capital ship in the game and keeping it balanced. All players get the same number of Dreadnoughts, as long as don't lose their Dreadnought factory. This issue has been discussed previously in this forum if you are interested in details.
5. "I can not build any structures in the binary planet gravity well. Was this by design?"
This is intentionally as you can not manually place defensive structures i.e making the Binary difficult to optimally defend.
"Again, great mod and keep up the awesome work. I'm looking forward to the next update."
Glad you are enjoying the mod and R4 coming soon.