I cant comment on any immobiliser problems if for instance you need to remove the battery to reach a 240v power socket.
What do you mean by taking 20 seconds to start? If battery was able to crank the starter reasonably powerfully for 20 seconds before the car started the problem may lie with the car, not the battery.
The resting voltage of a fully charged battery should be 12.6V . 12.41 Volts suggests its partly charged but might still be healthy. .
If you typically only do short journeys even a 40 minute run may not be enough to fully charge it. With the engine running you should be getting more like 14.4 volts.
I also had an asian made Original equipment battery (not Yuasa) that lasted more than 9 years.