You have considered and asked much! It would be helpful if you would indicate the depth of the canyon, the distance of the forays, and whether any forays will be on top.
Canyons are usually too irregular to enable acceptable radio coverage from the top or the bottom. They are like caves without roofs. The material (rock) is resistant to the coverage filling multipath that cities have. Even the water surface is not helpful. The reflections and refractions are not there. You would need lots of power and/or height to achieve radio line of sight and significantly counteract the losses. The most enhancement (however little) will be from fixed equipment on top. The BEST enhancement (however more so) may be from MOBILE units on top travelling along with the pack down below (if possible), even considering not using the tall masts.
Regardless, ALL users must be licensed amateur radio operators. Stay away from weak signal, satellite, simplex, etc, frequencies. "Coordinate" all frequencies with the affected ham radio frequency coordinator(s) because you never know when someone coordinates a permanent system within range which may or may not be on the air. They'll want to know where skip is coming from if there's nuisance interference and the callsign of the repeater when it is being identified.
If the treks are short, use only enough power to reliably maintain communications. Using lower gain, even unity (0dB) "gain," antennas on top will help coverage in the canyon at the expense of range on top. Those two rules will also reduce interference within the system.
Everything is SO much simpler and less expensive at the fixed site if everything takes place on UHF. Since not all field radios have UHF, you desire VHF, AND 600 kHz offset (hopefully NOT the 6 kHz in your post!) at that. Not only that, but you desire the cost to be far less than $1000USD.
From a KISS standpoint, you might consider using two radios capable of crossband repeat as you considered -- but with the following differences:
Radio #1: VHF receive on the system input frequency F1. Crossband UHF transmit link F3 to the system output site. Distance separated with the output transmitter power on F2 kept low enough to keep the output transmitter from desensing the receiver too much. There will be an acceptable amount of desense. Preferably the VHF receive frequency F1 should require Tone A. The UHF link on F3 will avoid the need for 800+ feet of transmission line. It only needs to transmit in one direction! A crossband mobile diplexer (receive and/or transmit simultaneously on two different bands - not a duplexer which is single band only) is smaller than the radio and is inexpensive or comes with the radio. Single-band or dual-band mobile antennas with a ground plane are inexpensive. Diplexers are installed at the radio and usually are not dust and moisture resistant as the radio may be.
NOTE: For best results, with different height masts, Radio #1's antenna (the F1 input) should be at the highest height. Radio #2's F2 output power would help counteract the lower height difference. Most of the isolation will be from horizontal separation. Vertical separation isolation generally occurs when one antenna is above the other for the distance required to achieve the required isolation.
Although the link F3 could easily be miles in radio line of sight without beams, the F1/F2 coverage and/or range disparity will probably be unacceptable at that distance.
Radio #2: Receives the UHF link F3 signal from the other site and crossband repeats it on the VHF system output frequency F2, preferably with Tone B. VHF receive at the other site will still suffer some desense, but noises won't show up on the link because the tones are different. Tones are categorized in groups. Research so you do not pick tones in the same group. Normally this is done for reducing interaction between co-sited public-safety/commercial systems. Tones from different groups, in your case, will further reduce system noises. Note: Radio #2 will not interfere on the link frequency F3 with Radio #1 IF people using simplex on F2 use Tone C. Therefore Radio #2 will not and should not ever transmit on F3 - if the system is configured properly.
Hopefully, I've remembered how to do this much less expensively than I normally did decades ago (not necessarily for ham radio).
Or you can get fancy for a little over $400USD:
https://qrznow.com/portable-repeater-controller/amp/Note that filtering and radios are also needed for the fancy system.
Mike WB0DZX