Of course you could use rpi as gateway, but as personal advice I'd suggest to get some mini pro as sensor nodes and keep the Uno as the gateway or the prototype platform. The most flexible solution would be to make an mqtt ethernet gateway and use rpi as mqtt broker and controller with openhab or whatever else you like, since the mqtt in the middle allows also to have multiple controllers. I also wanted to try openhab and tried openhabian but it turned out to be too complicated, so I moved to domoticz and it was much easier to put things together.
Remembering now, the undervoltage and overvoltage protection could both be put under the MCU's control, in which case you could get by with just a diode, or equivalent. It wouldn't be as failsafe in the sense that an unforeseen bug in your code might defeat it, but it could be done. In the end, I'm not sure that the chance of encountering a failure in software is necessarily any higher than that of encountering a failure due to defective hardware.
Alternatively, you could put two cells in series, and in principle that would eliminate any chance of overcharging. With that configuration, if you used a 5v solar panel instead of a 6v, then you'd also eliminate any chance of burning out the atmega328p and wouldn't need extra hardware to ensure against that.
For that matter, you could stick with one cell and just use a 3.5v solar panel. With just that one stroke you wouldn't need anything extra to guard against overcharging the battery or over-volting the atmega328p.
That's what makes embedding solar such a fun problem--depending on the use case, there are such a large number of different ways to solve it. So many that the real game is in comparing the trade-offs.
@skywatch cheapest price I could find is 4,1€ by the way incl. VAT + shipping. I guess that's a good price - especially if you consider that you don't have to buy thousands of it to get that price.
A quick search revealed the LPC4300 series (LPC4310 e.g.) from NXP. That one has a Cortex M4 and a Cortex M0+ inside.
How would you solve things like certificate updates / TLS connections on a gateway with an MCU?
I use these: https://www.aliexpress.com/item/32855681042.html Well, they work, but profile is not the best for dupont (2.5mm). But for their price it's ok.
YEFYM SN-2549 crimping pliers 0.08-0.1mm2 28-18AWG 2.54 spring 2510 terminal =SN-28B+SN-01BM eupop style wire crimp tools
The latest addition to my coin cell nodes, a PIR motion sensor based on the Ebyte NRF52805 module.
The PIR sensor is a Senba S16-L221D-2. I'm measuring around 24uA, which is higher than advertised, but with the 6-7uA from the NRF52805, should still get me 3-4 years on a single 2477.