OTAA and device power-off

I’m working on a device that takes measurements every about 5 minutes. The idea is to make it work from a battery and a small solar panel. However, the board I use is not the most power-efficient (it takes about 100 mW while idling), so I plan to switch it off between the measurements using an RTC.

The problem is that this triggers OTAA since it’s a cold start. And if I understood correctly, it would also prevent me from using ADR.

Is it possible to avoid activation on every start-up? Say, use OTAA only once a day instead of 300. I have non-volatile memory, could I use it to store whatever important parameters and restore them on start-up?

Edit: I think I figured that out. The library I use (LoRaWAN for Pico) supports NVM.
I had to correctly set it up on my device and then go straight to sending instead of trying to join.

However, I still have a question left. Should I invalidate the connection if the device was offline for some prolonged time?

No need to re-join. You can join once in the lifetime of the device. In fact, it’s not a good thing to re-join. Downlinks are “expensive” for the network.