60x16ft round aluminum tritoon river project - any Pro?

Dear boaters,

First sorry to launch this subject again. I read a lot about why designers don’t like the round aluminum pontoon here.
I understand it is less efficient than a v-shape at some speed, I read that it can be dangerous because of the added buoyancy getting exponentially lower when more than half the tube is in the water…

My point is to compare that type of hull in a very specific case:

I want to build a river boat to use on La Seine, the French river that crosses Paris.
I’d like it to be big enough, not to live in, but for a multi-day cruise in Europe. It is why I came with the 60x16feet, about 12 metric tons once loaded.
It will have big windows to have a panoramic view on the canals, rivers, cities. That kind of boat.
Speed is definitely not something in mind here. In Paris the limit is set at 12km/h, and it is 18km/h outside of Paris.
Total cost would be around $100k, and no more than $30,000 for the hull.

The options I have found for the hull are:

- Convert an old river boat (frecynet, tjalk, luxemotor) and keep only the hull. I would find it sad to damage what remains of our past river transportation industry. Plus those boats weight around 100 metric tons, mainly due to their heavy steel hull. The extra power needed does not make it an environmental friendly solution for my need, and maybe not an economically viable one.

- Find some catamaran hulls, which have less drag and so are far more efficient. Problem here is price. Catamarans that size are way out of my budget. Not sure to get what make them so expensive on a construction point of view...

- Order a 18mx5m round aluminum tritoon kit (tubes diameter 90cm, 4mm thick) and build on. I received estimates at less than $30,000 for that, which match what I can invest in the hull.

My questions to you are:

-1- Is that right to consider that the drags will go as follow: 12 tons Catamaran < 12 tons Pontoon < 100 tons old heavy steel hull ? (I start with an easy one!)

-2- Is that right to consider that at 18km/h the difference of drag between a nice Catamaran hull and a round aluminum tritoon is not so critical and would not justify the extra $ to get a Catamaran hull from a gas consumption point of view, maybe unless it is a professional use with 8hours / day during 10 years or so?

-3- In conclusion, could the round aluminum (called ‘cheap and dirty’ by some of you) make sense in my case? (affordable, 60ft 12metric tons, 18kmh)

-4- Some passenger boats generate waves of 3 feet or even a little more in a busy night in Paris. Would the pontoon be safe? (I plan on having a buoyancy that doubles the overall weight when loaded)

I know I might ask some basic questions here, I actually ordered a design book but haven’t received it yet – so thanks a lot for your answers!!

Arn0