with things like nylon, printed in a heated chamber you could possibly do it.
Layer bonding in nylon is excellent - but it's a real bastard to print.
The formlab website states that the chamber only gets to 45c - in my opinion that's not hot enough to get a watertight print with nylon. Even for abs, ideally you need the chamber to be around 60c to make it worth your while.
For things like polycarbonate, nylon and some of the other newer industrial materials - it needs to be hotter still.
Polycarbonate also needs a hot end able to run at around 280c for long periods of time, plus it won't stick to standard printing surfaces.
I've got two 200gm rolls of polymaker pc plus that I can't really do anything with.
If you're in the uk - happy to send you one for p&p.

Density isn't the issue. It's purely down to layer bonding and the fdm process.
No matter how dense it is - it's still printed one layer at a time - hot plastic to cooler plastic. It's the temperature based shrinkage that stops fdm from producing things with the properties of moulded plastics.
So the hotter the print chamber - the better your layer adhesion will be. and as the chamber cools the whole model in one go - you don't get variations of shrinkage due to varying temperatures.

If it were me making a pressure vessel at anything like serious pressure I would most likely wrap it in thin resin fibreglass tape. Just to make sure.
If you could modify the printer to take PEEK - You'd be more likely to have real success. But it would need the hotend upgrading, probably need a better heating element on the hot bed and you'd definitely need a hotter build chamber with thermostat.

If you've got working models printed on a form 2 - then that would probably have been your best bet. There is a pretty decent variety of resin types available now as well.
Layer bonding is close to moulded and they are actually better for awkward shapes and geometries.

The other thing to consider would be the new formlabs sls machine: The Fuse 1
Laser sintered powdered nylon. Parts take a lot more post processing and might not be suitable for something that needs to be hollow and only has small in and out channels - as it would be filled with unsintered powder that would need to be extracted.

But parts are super strong - downside is it's £10,000.
https://formlabs.com/3d-printers/fuse-1/
Having handled things it's made I'd say results were as good as machines costing 2 to 3 times as much.

It does sound like you might have been better off with a form 2.
Resin prints are going to be a lot more like cast plastic yes. It's a much higher resolution print and the layer bonding should be near perfect as it's wet and solidified rather than melted in - relatively - thick layers.

The thing is to try different materials and see what you can do with what you have.
But in all honesty I don't think you were sold the right machine for what you want to make.