dRehmFlight open source flight software for teensy microcontrollers
not the same MCU I'll be using, but probably a good reference.
developed using arduino IDE and C++, so it is a much more similar (and simpler) flight software to compare/reference than ardupilot or betaflight.
already been proven to work for VTOLs, so a good resource to see how he handled transitions to and from vertical flight
ESP-NOW
ESP-NOW: "ESP-NOW is a low-power, peer-to-peer wireless communication protocol by Espressif that enables fast, connectionless data exchange between ESP8266, ESP32, and similar devices without Wi-Fi, with a typical range of up to 100 meters in open space"
could be a viable alternative to the xBee - easier to setup/integrate with arduino? There's a lot of arduino support, its cheap, and has plenty of range