Z-Stack-firmware VS com.zsmartsystems.zigbee

cc2530, CC2531, etc.: I found many using/reflashing these as gateways or routers and there is a repository of firmwares for that, but did not find anything about using them to build implement end devices (sensors/actors).

It'll be one of the ones listed here: https://github.com/Koenkk/Z-Stack-firmware/blob/master/coordinator/README.md

Have you read the readme in the coordinator folder? That is probably the info you're looking for: https://github.com/Koenkk/Z-Stack-firmware/tree/master/coordinator

Is it possible to use the Sonoff Zigbee 3.0 USB Dongle Plus-P as a ZigBee sniffer? I downloaded Packet-Sniffer-2 and flashed it with the cc26x2r1lp/sniffer_fw.hex (at first using cc2538-bsl.py, then later using TI's Flash Programmer 2 and the cJTAG pins), but the Sniffer Agent doesn't see it. TI's supported hardware list includes the CC2652R, but not CC2652P, but I was hoping they'd be similar enough that the same firmware would work. But on the other hand, the Z-Stack firmware for the two chips are different.

Sonoff Zigbee USB Dongle 3.0 based on TI CC2652P+CP2002N (flashed with the latest firmware 'Z-Stack-Firmware CC1352P2_CC2652P_launchpad_coordinator_20220219.zip for coordinator' using 'TI flash programmer 2',

ZigBee can do way more than 32 devices depending on what coordinator you use. With a modern cc2652 based coordinator you can do 100s of devices. See the routes column (and explanation) in the table on https://github.com/Koenkk/Z-Stack-firmware/blob/master/coordinator/README.md

I succesfully flashed the stick (CC1352P2_CC2652P_launchpad_coordinator_20220219.zip)

This device just plugs in and acts as your coordinator. Here is the latest firmware if you need it the factory firmware may be sufficient: https://github.com/Koenkk/Z-Stack-firmware

Little update: This Java library looks promising. Bummer it doesn't support ZigBee 3.0, but it looks a looot easier than trying to implement the entire spec from scratch.