I think you've got the concept - run under Part 90 as an FB6/IG - but you need to certify that you're going to provide communications service to "Part 90 eligibles" - which means US Citizens and US Companies that would be otherwise eligible under Part 90. Technically - you could also provide FB6 Private Carrier service to Public Safety entities - or a non-profit Town Watch group.
I'm not the law, and I'm not the FCC, but I've done this type of licensing for other entities. You are the one who needs to certify what you are planning to do, but I'm not aware of any requirement to "show your books" to the FCC to prove that you're running a for-profit business, or any site inspections. You will need to run Part 90 type accepted equipment, and you will need to certify buildout/construction within 1 year of your License being granted by the FCC.
Judging by your GMRS license, you're located in a fairly populated area, so finding a decently clear frequency pair might be a bit of a challenge in the Part 90 spectrum. If you've already got a 60 foot mast, try to monitor some frequencies and see if you can identify any open frequency pairs in the area. Used to be that some of the older 451.8xxx pairs were clean, but most communications companies have snatched those up - at least in my area.
I'd try looking up someone like FIT (Forest Industries Telecommunications) or EWA (Enterprise Wireless Assocation). They'll walk you through the steps. I find FIT to be a little more personable - but EWA is very professional & capable. Up front cost isn't cheap to get the 10 year FB6 license & coordination - expect to see something around $700, but it's still pennies per day. If you tell them you're looking for an FB6 UHF repeater pair with DMR/Trbo emissions, you'll be off to the races.
