Just realized I sort of got off on a rant and didn't answer your question regarding other gauges! Monitoring key parameters is a good idea and something we always did on experimental engines to catch problems before they caused a failure. Sometimes it even worked!
Did performance work on jet engines for the first 10 years of career at Pratt, and became a bit of a fanatic on instrumentation. It used to drive me nuts when guys would use a 500 psi transducer to measure 30 psi, when they should have used a 50 psi 'ducer! Accuracy is always a percentage of full scale reading, so smaller ducer was 10X more accurate. This is why I prefer mechanical gauges with their 270 sweep where you can use a large scale, instead of the small, compressed 90 degree scale on electrical gauges. Electrical gauges are less reliable, especially on a boat. Digital gauges can provide tons of info, but take some getting used to. Our brains don't process digital info as quickly as position of a pointer on a dial! That's why you don't see many digital gauges in race cars!
The vacuum, water pressure, oil temp and fuel pressure gauges will give you a good idea of what's normal for your engine. Helps to stick a small piece of green tape on gage at "normal" pointer location; then a quick glance will pick up a potential problem.
We used audible alarms for the really critical stuff on the Pratt engines; they can be a pain, as they go off when key is turned on, but many industrial engines use them, so could consider for oil pressure and water temp. Only other thing I can think of is some way to monitor oil temperature in lower unit, but don't know if Volvo makes anything for that; might check the aftermarket.
Be careful with the plumbing on that fuel pressure gauge! I believe Autometer uses a diaphragm with theirs to minimize amount of plumbing with gasoline in it. The diaphragm goes between the fuel pressure source and the tubing running to the gauge, which I think is just a very sensitive air pressure gauge, but calibrated to read fuel pressure.
That sounds like a very efficient power package in your Seafari . . I'm sure the Forum members would be interested in some performance numbers, especially those considering a conversion to outboards! Denny