So, this "project" was something I poked at between spring and autumn of 2021. Can't remember everything, but hopefully this is enough for you to do the same. I'm not experienced in electronics or Arduinos so you might notice that.
Anyway, watched the Hackers movie about a year ago and after that I had to have a Motorola Advisor pager. Bought one on Ebay and once it got from Kazakhstan, though Customs and all, to my hands, I noticed that it was on a marine VHF frequency. Luckily you can pull the radio receiver board out easily and put different one in, but those things are rare around here and/or I'd need a license to send messages... Then I had this MotoLoRa brainfart, which I'm trying to document here.
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Back to business. I looked in your code, and I think I fount reason why Arduino hangs and need to restart. F5OEO Evariste (author of rpitx) wrote code for Linux. It is OK to use malloc() realloc() in Linux. He incorrectly do realloc() and forgot to free() memory after allocating it. Linux handle this problem when program finish and clean allocated memory. But memory management is not happening on Arduino like in Linux. Also F5OEO rpitx code is memory hungry. And result is Arduino code crush. I am working on Raspberry Pi Pico/AtTiny84 code from almost scratch but without serial config menu for now. I also copy/paste F5OEO rpitx POCSAG code and optimize it for Arduino. Hope will finish code before my gadget arrive after 2 weeks.