Ah OK, then the Doona, Jané seats and the Cybex ones won't work for you (unless you get the base for the Cybex).
The Kiddy seats were brilliant but are unfortunately no longer available to buy.
So you would be looking at Maxi Cosi Marble, Britax Babysafe 5Z2, Joie i-Level or Nuna Arra (or Cybex Cloud something with base) if you want the recline feature where the seat itself goes a bit flatter and this can be used in the car.
Or something with the in-built padding making the seat flatter for baby if that is something you'd like to look at. There are probably more of these than I'm aware of but these are the ones that I know which typically work really well:
Britax Baby-Safe: i-size / 2 i-size / 3 i-size, or i-Sense
Avionaut Pixel, Pixel Pro and Cosmo (also Venicci Ultralite, as someone else mentioned)
Maxi Cosi Pebble: Pro, Plus, or 360, also Maxi Cosi Coral.
I can find info for you about how each insert works as they aren't that clear on their product websites etc.
One issue with these seats with an insert is that most newborn inserts are designed to be removed when babies are approximately 60cm which is around 3 months old. Your baby being 3 months at the time of the big trip means it's uncertain whether you'll still be using the newborn insert at this time. Usually, there's a bit of flexibility and you can keep them in until around 4 months but if your baby is very long in the body, then they might not physically fit. On the other hand, if they don't need the newborn insert any more, then arguably they also don't need such a flat angle any more either. 3 months is a bit tricky of an age because it's more like 4 months that most babies have the really good head control e.g. can hold their own head up if you prop them in a sitting position.
There is an amazing looking seat coming out next year by Besafe, I saw a video last night and thought instantly of this thread, but I assume that will be too late for you. They don't actually have a release date yet but I have heard "Spring 2024".
Lastly I haven't included any longer lasting seats, but know of a couple which might be worth looking at if that was something you are interested in. I assume that you're wanting something that is compatible with the pram, though?
What I'd say to do if you have an option at all is to look for any large and/or independent baby goods shop near you, (to find one, look for e.g. Maxi Cosi or Britax stockists on their website and look for non-chains) and see if you can go there and have a look to see if they stock any of these models and you can physically try them out. If they happen to have a weighted, proportioned demo doll based on a roughly 3 month old baby (this is the 6kg doll) then that's really helpful as you can try out the doll in various different seats to see how they look and fit. You should also be able to try the seats in your car. This is helpful especially if you're able to measure the relative angles (e.g. spirit level app for smartphone). As often the shape of your seats and position of your isofix will make a difference in the individual fitting of the seat.
Then just some personal experience - we live in Germany and tend to drive back to the UK which is about 10 hours' total driving including a ferry trip (of course on the ferry you don't stay in the car, anyway.) We did one of these trips when DS2 was about 4 months old, at his first Christmas. DS1 would have been about 10. We had a Cybex Aton 5 car seat which uses a small polystyrene wedge under the baby to push them up into a flatter lying position, which wasn't that great for a tiny brand new newborn, but by the time of the trip it was OK and then we removed it anyway shortly after that. We did most of the driving at night. We did up to 3 hour stretches, maybe a single 3.5 hours and then I started to get too antsy. With the advice being 2 hours I wasn't that keen on pushing it, but 3 hours worked well anyway, as we all needed toilet and leg stretch breaks every so often, and at that age babies often still wake every couple of hours at night to feed, so it wasn't like we could just keep going for hours and hours and hours anyway plus it probably wouldn't have been a very good idea for the adults. What we did was just try to keep most of the stops very brief, literally just a toilet break, quick feed in the car and then back in the seat and back on the road. Depending on where your ferry break is in the journey, that might help too being a good long time out of the seat. We found that towards the end of the journey more frequent stops were necessary, so we were concentrating most of those longer stretches of driving to the very start of the trip and trying to cover as much ground as possible, which is still our approach now the DC are older.