Last century Volvo used similar lift pumps on most small engines but with different diaphragms depending on fuel used ie either petrol or diesel which would suggest to me the same allowance should be to pump bulbs
Lack of Gearbox oil pressure especially after oil change is first area of thought
As this thread has been around for some time perhaps new thread may help and widen scope
Used it for years
It has evolved into an excellent app and appears to be managed by an active boater
I am no longer active boat wise and advised Stuart and had a excellent conversation with him
All the better that it runs on phone and can live in your pocket
They do suffer from a carbon build up round the heater plug but this is easily cured by carefully removing the plug and using a twist drill turned by hand sorry can’t remember size
Do a vacuum test on the lower unit and consider replacing diaphragm as normal practice at this vintage
You have covered the main causes from the simple replacing the drain plug washer on a yearly basis
Checking the prop shaft seal are not damage from having picked up something that displacing...
May be de rated (for commercial purposes ) and used for pleasure purposes hence the explanation given
Check spec number with Volvo to confirm
Worth putting it through a prop calculator