Ideas of maintenance tasks on my 360 engine?

Clean or replace injectors.

 

There is a regular major service every five years that you should follow. Timing belts, tensioner roller bearings, spark plugs, cam seals, valve cover gaskets and cam timing. Every so often, there is a leaky water pump and heat exchanger to replace. Headers with cat material breaking up need to be replaced eventually. O2 sensors don't tend to last real long as is the secondary air injection pump. CV boots do break often under the intense heat. Ignition coils need replacement. Sometimes, cam variators need replacement.

Nice picture, by the way.

 

3-year cycles for major service is quite overkill, but it's your car.
Cam seals and valve cover gaskets never replaced, even with timing belt job? How do they time the cam shafts? Don't they leak by now?
Secondary air pump does not affect performance but its failure throws CEL code for o2 sensors and air pump. In the US, that gets you a smog test fail and you can't register your car.

I would say that ... IF you have leaky valve covers or cam seals, you should change those out at the next timing belt job. Also insist on a cam shaft re-timed to factory spec at the same time.