1) The linking bundle provided by myGMRS.com (https://shop.mygmrs.com/collections/repeaters-and-accessories/products/repeater-linking-bundle ) is quite capable of this with a few modifications to rpt.conf and iax.conf you’ll be on your way.
2) Allstarlink has a download page using the ASL image for the RaspberryPi. In your situation I would consider the v1.01, and follow instructions from the ASL website links etc.
3) I would suggest the myGMRS route due to the complexity of Linux, and Rich went through great lengths to keep it clean, and LEGAL!
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