They are still relatively new. They may be a good choice but also not. I also don’t expect that there will be Arduino support for it soon and if there would you would still need LoRaWAN library support for the SX1262 which in Arduino land is still very limited. As you said you are not a programmer STM32L currently would probably not be a good choice for you.
All really depends on what your requirements and goals are.
If your goal is to learn about LoRaWAN and TTN then start with something simple that just works. Spend $15 to $25 on a board that just works but is not perfect, is popular in the community so you can get some support, doesn’t cost too much and when you outgrow it and are more aware of your needs then you can make a more weighted decision for your next step(s).
Have a look at the Big ESP32, CubeCell, Big STM32 and T-Beam topics on the forum to get some impressions. Adafruit has a Feather M0 LoRa board which is SAMD21 based, but you won’t find much info about it on the forum. (A disadvantage of the Adafruit is that it does not have an antenna connector and only allows to solder a piece of wire as antenna.)
The more clear your requirements and questions are, the better others may help point you in the right direction.
My advice is to pick a LoRaWAN development board that is either supported by LMIC-node (because LMIC-node supports most of the popular LoRaWAN development boards that can be used with the Arduino framework and the LMIC LoRaWAN library) or get a CubeCell board.
Pick at least a 32-bit board and not one of the older 8-bit (ATmega328 or ATmega32u4) boards Some 8-bit boards are supported by LMIC-node but should not be used for new development because of their limited RAM and flash memory (but you are already aware of that).
(CubeCell is not supported by LMIC-node because the LMIC library does currently not support the SX1262 LoRa radio.)