Hmmm....that, 500 yd shooting and a 6mm don't really compute. Loprs of guys supporting your choice, but frankly, I think a 6mm is way too small.
25, maybe....6.5 better....6.8-7mm are ideal...30 is more versatile, but a touch oversized.
My gut feeling is rounds like 6.5 prc, 6.5x284, 6.8 Western, 270 WSM (throated for bigger bullets), 280AI would all be nice longer range deer slayers.
Also, I would spend some time finding a gunsmith that speaks your language. I have a buddy that dropped good money at GAP, and is very satisfied.
There are basically 3 kinds of gunsmiths, IMO. Full service, I listen and build what I think I heard you say you gunsmiths. Package guys, willing to build full custom, but basically build from packages. ....do it all guys....these guys often have the best experience. They seem to take almost any job as a challenge, but you really have to tell them each detail you want.
I'm sure every gunsmith is an individual, but this was kind of what I've seen. For a first high end build, a full service place like GAP, makes a lot of sense.