The limitation is mostly in the software of the base band modem, which is separate from the OS.
iDEN Direct Talk is the closest thing I'm aware of. It used the FHSS on the 900Mhz ISM band. If you get phones(probably off ebay) that have SIMs that were previously activated, it is still possible to use Direct Talk.
I haven't personally used these since the iDEN network was shutdown, but there are a number of videos on youtube.
I do have some Motorola DTR radios, which use the same FHSS on 900mhz ISM (didn't keys, so they are not compatible) Performance I see is about 1 mile through buildings/tress, but not large hills.
They work extremely well.
This is just radio to radio, there is no relay, so messages/voice isn't forwarded, though this could be done with software.
Which gets back to the modem software. On phones, this is different than the OS, and in most cases very locked down, you aren't allowed to alter the firmware. Cellphone companies specifically don't want non-authorized(locked down) modems on their networks.
Next thing to keep in mind. Cellphones all operate on the license of the Cellphone network operator(Verizon, AT&T, and T-Mobile)* In the US, you are required to have a license for most Radio transmissions. So if you were to somehow modify the modem firmware to work as desired, you wouldn't be operating in the way the license holder wants, and they could send the FCC after you.
In the US there are a few bands that are open to use without a license(though there are restrictions.) CB(No data/encryption), FRS(No data/encryption), and ISM( 900mhz, 2.4Ghz, and 5.8Ghz), and CBRS(requires you to register, and check a Database controlled by google before you can actually transmit).
2.4ghz and 5.8ghz are used by Wifi. 900mhz is used by the Motorola DTR, LoRa, and a few other things, less common, but usage has been picking up a lot recently thanks to LoRa. Of these, the 900Mhz.
CBRS(3.5ghz) was extremely promising, but thanks to the lobbying of the Cellphone networks, including google, you can't just use it, there is a complicated process and spec you have to abide by in order to do so, which is out of the reach of most citizens, despite that being the original intent of the band being opened up.
* As mentioned above, cellphones operate on the frequencies that the cellphone networks have paid for, the exception to this is CBRS. A lot of newer phones have support for CBRS because the carriers themselves intend to take advantage of this frequency.
CBRS is probably the easiest to move forward with using existing cellphones. The downside, would be the requirement of checking the google DB, and getting the permission to transmit before doing so, which complicates the whole p2p, and off major network problem. There is base station hardware available, though I don't if you could install custom software on it. Most of it is from the Helium crypto/cellphone network.
I also want to comment on this: "They are stronger, in terms of bandwidth and throughput." In most cases the cellphone radios are not any more powerful than Wifi, or BT. The main difference is the frequency(Simplified: lower frequencies reach further with the same power), and that the Cellphones communicate with raised towers. Most radio communication is line of sight, or close to line of sight(though there are exceptions such as bouncing off the atmosphere.) With the DTR radios I mentioned above, I have reached 4 miles(the longest line of sight I could get in my area). They are 1 watt. Most phones are limited to 1 watt.
I use Watts when talking about power, but the FCC regulates transmit power based on effective isotropic radiated power (EIRP), which is a combination of the antenna and power you use. so sometimes the transmit power will be higher or lower, depending on the antenna used. Most wifi usage falls within the same range as cellphones.
For p2p stuff, I could encourage you to looking into LoRa devices. Meshtastic [https://meshtastic.org/docs/introduction/] is the software most are using. It is great for text messaging, but doesn't support voice(yet)
This was a long comment, hopefully it was useful, if you have additional questions I can try to answer if I know, feel free to reach out.
