Do I need to replace my batteries?

[Paramotorgreg]

I have been reading a lot about batteries on this forum and it is clearly a very complex subject - there are some quite different and sometimes contradictory points of view on how to get the best life from them and how long they should last. I am therefore not expecting simple answer, but I would appreciate and be grateful for some expert opinions.....

I have 4 Varta Professional 180Ah 930 180 100 batteries as my service bank. I also have one Varta Promotive 180Ah M16 680 109 100 as my engine starting battery. I don't know exactly how old the batteries are, I inherited them with the boat, but I have owned the boat for 3 years. I charge the batteries using shore power or my generator when the service bank voltage drops to 11.9v (I don't generally monitor the engine start battery voltage, but typically when the service bank drops to 11.9 it is higher at ~12.6v). I have a Mastervolt Mass 12/60 battery charger. When I charge on shore power I charge continuously. When I charge on the generator I charge for approx 1 hour in the morning and 1 hour in the evening or until the service bank voltage shows 13.5v.

Generally the only significant load on the service bank is the fridge and my understanding is that there is no load on the engine start battery other than starting the engine or generator.

On a couple of occasions recently I have started the engine in the morning then sailed all day. Although in the evening the service bank is showing 12.0/11.9v and the engine battery has been turned off all day, when I have tried to start the engine there has not been sufficient power to turn over the engine. Fortunately there has been enough power to start the generator so it has been necessary for me to start the generator and charge the batteries for a few minutes (5?) before I can start the engine.

My questions are several......
1. Am I discharging the batteries too much?
2. Am i not charging the batteries enough?
3. Are my batteries at the end of their useful life and do I need to replace them?
4. Is there anything I can do to increase their longevity now?
5. If they need replacement, what can i do to the new ones to maximise their life?

All advice and information grateful received, and preferably with the minimum of technical language, because as you may have gathered I don't have a huge understanding of this subject.