Meshtastic

I don't think what you're trying to do is very practical.

For US 915, there are between 52 (SHORT_TURBO) and 208 (LONG_SLOW) frequency slots. Each node has to use the same LoRA parameters (called mode presets in Meshtastic) as well as the same frequency slot in order to communicate (or relay).

LoRa signals are low bandwidth (125, 250, or 500 kHz) so there are a lot of frequency slots. In addition to having one radio for each frequency slot, you would also have to have one for each LoRA parameter preset in that frequency slot. Nodes can only operate in one mode and frequency at a time.

So to cover all cases, you'd need:

• 52 nodes operating in SHORT_TURBO mode
• 104 nodes operating in SHORT_FAST
• 104 nodes operating in SHORT_SLOW
• 104 nodes in MEDIUM_FAST
• 104 in MEDIUM_SLOW
• 104 in LONG_FAST mode
• 208 in LONG_MODERATE mode
• Plus another 208 in LONG_SLOW mode

Source for those counts

That's a total of 988 nodes to cover all, and that's not even getting into the fact that they'd be talking over each other.

Basically, if you want to be a generic repeater (note the docs clearly state to not run repeater mode willy-nilly), it's best to stick with LONG_FAST, set your custom channels as primary (if you even want to have custom channels, that is), and create a LongFast secondary channel with the default dummy encryption key. Then manually set your radio frequency to the one used by the default LONG_FAST setting (908.875 MHz).

That's basically what I do and lets me have my own private channels while also relaying for and being able to make contact with anyone else using the default config.

Edit: Coordination of local frequencies, etc is usually done by forming a Meshtastic community group. Then you could just join (or start) that group and use a standardized frequency and LoRa parameters.