[hiccup]

A couple thoughts.

Installing an inverter correctly is a significant undertaking. You need to tie into both the AC and DC electrical systems. You need to plan out how it will be wired (e.g. what AC circuits will or will not be powered by the inverter), how does the shorepower and generator supplies interface with the inverter? It really depends on how the boat was set up from the factory. I have seen Tiara do it many different ways. I don't think 40 hours is a bad estimate for a truly professional job that is done correctly.

The $4k for Victron stuff & shop supplies seems high but I'm not sure what was spec'd. You should be able to get a list of what was quoted and check internet prices to see how good or bad they're treating you. Copper wire etc is crazy expensive and there will be a lot of very heavy gauge wiring added to the boat.

I would be looking at adding a combo inverter/charger that would only draw from and charge the house bank and a separate charger that only charges the engine bank. This is typically how Tiara sets them up. I would also reach out to Charles and see if they can supply you with the specs on the Gel charging settings they have programmed into the charger. The settings may very well be compatible with some/most/all AGM batteries. I would also consider using LiFePo4 batteries for the house bank depending on how much you're going to ask from the inverter. I know, more $$.

Lastly you might look through the schematics supplied by Tiara for your boat. It is likely that they include optional schematics for the inverter option. This information could be leveraged for your install.