If your start battery is not starting the engine it probably needs replacement. It would be shame in the long term to fit the 80w panel but send its output to the start battery. It would do far more good fed into the house battery.
The 5A Jaycar controller is a "bang-bang" or "on-off" controller. It is not PWM. These are very inefficient and importantly for a start battery struggle to get the battery up to 100% even when connected for a long period of time. It is worth spending a little more money
and getting a PWM controller.
I think you have copied the wrong link for the MPPT
controller, but I am not a fan of cheap MPPT
controllers. They have a high failure rate and don't track the the Vmp well. They often sacrifice important features like adjustable set points for a poor MPPT tracking.
There are some exceptions the Genasun range of controllers are reasonably inexpensive they have a well implemented MPPT tracking system. The set points are not user adjustable, but they are bit more sensible for a boat with a 14.6 boost voltage (although 14.4v absorption). I believe for a small extra fee you can get the factory to adjust the set points. However there largest model is only 10.5 A which does not suit your solar
panel size even with multiple controllers.
It sounds like you have plenty of room for solar
in which case it is better to put your money
into more solar wattage rather than MPPT.
Plasmatronics make a range of well made PWM regulators and would be worth looking at.