Looking for a wireless intercom system with local power supply

Hello everyone,
I would like to install a mailbox with a doorbell and intercom at the entrance to our property, but I don't want to run cables all the way there at all. My requirements:

[*]3 doorbell buttons
[*]DECT connection for intercom linked to FritzBox
[*]local power supply (battery + photovoltaic)

The distance from the intercom to the FritzBox is about 35 m as the crow flies, the wiring route would be about 100 m.
The second-best solution would be to lay only power there, then the "doorbell mailbox" would be moved to the other side of the street, where it is somewhat less favorable.

Any tips?
Thanks in advance!

Do I remember correctly that you will have 3 internet connections?
Is the phone only working with you (so everything through your Fritzbox) or do the children also have a phone?
Wi-Fi is not a stable alternative at that distance with walls (unless you have additional access points).
So many doorbells are already out of the question. Often they only have one button and you would need three of them.

DECT would be possible but all stations I have seen only have two buttons.
Did that get overlooked in the planning?
Run a cable. If you are a bit willing to order, take a look at KlingelPaul (UK Intech).
Get 3 buttons and connect them with KlingelPaul.
If you want it future-proof, also lay CAT. Then later a camera or doorbell station with POE is also possible. The cables don't cost a fortune.
Later you will only be annoyed if you want to change something. Now it is still a comparatively small effort.

Without cables, it will just be junk or nothing you can rely on. A Cat cable and, for example, a 2N Verso IP doorbell powered via PoE would be sensible. It calls the DECT devices through the Fritzbox via SIP.

I already feared not to get around the cabling, no misplanning - just some laziness. So CAT7 and 5 x 2.5 for pathway lighting and "power at the mailbox doorbell." The KlingelPaul has something, but unfortunately no intercom function.
We will have 2 lines, the guys share one. The intercom system with bell can be routed to a Fritzbox, then immediate call forwarding to the guys' phones.
I have now set my sights on an Auerswald 90636, which can be connected via an A/B input.

Is it really 100 meters (at 35 meters visibility)?
Just use CAT8. Make sure to have good shielding.
If you’re going to do it "right" now, I would go for SIP and not for A/B input.

Yes, the path is so long because there is also a difference of over 10 meters in height, and I can guide the cable better along the access path and do not want to go straight down the slope. Our Fritzbox is in the house connected via LAN on the west side of the house and thus closer to the beginning of the driveway again. CAT 8 then also makes no difference, I can use it.