GMRS- Yes it can be possible. Line of sight, repeater antenna as high as possible even that tiny repeater we have got 8 miles in bad locations.
If you are truely flat no buildings or hills in the way it is possible for HT 5W to get to it, but where I live it is not good past 2.25 miles.
The key is antenna height, for everyone.
Last I checked $35.
Also find the highest DB gain antenna for that tiny repeater and the highest location possible, with the shortest LMR-400 cable run you can.
Do not attempt to tie a radio to the repeater, get an HT for you to talk to it. A mobile for a vehicle.
May not have hit all your points.
Royce