Bonjour,
Welcome to France.
I would suggest to look at
https://lafibre.info/la-fibre/comparatif-fibre/msg1125031/#msg1125031 that will cover mainstream ISP.
In France, if you are in a fibered area, you can't get DSL anymore.
Most ISP offer 2 speed levels: 1 Gb/s is the standart, up to 8 Gb/s for high speed fiber.
Bundles that may be included: IPTV, unlimited phone calls, streaming...
Most of them also offer package with Wireless.
If you describe, what you need, we may direct you to specific plans.
Latest ISP not already suggested above, Free:
1. Does orange give you a non-CGNAT IPV4 address?
1 Public IP, static. Default is a shared Ipv4. But you can request a full stack ipv4 in a click.
2. Is it possible to order a fibre package with an ONT only, instead of their "Livebox". (I have my own router set up and I am happy with it.)
NO ( as a short answer )
3. Otherwise, is it possible to put the livebox into bridge mode, so I only use it as a modem?
Freebox offer a bridge mode. Also ipv6 delegation to sub router.
4. And finally, does livebox5 allow for "advanced" settings like portforwarding (including low port numbers like 80 and 21)?
N/A if you use the bridge mode.
Suggestions:
Série Spéciale Pop 24 EUR/m or Freebox Révolution Light 30 EUR/m