Strengthen a deck

This is a different deck to my other deck question.
I have an existing deck. The joists are simply 2 pieces of pine glued onto of each other spanning 3 meters. It is wobbly to walk on (and I don't know how it was ever approved).

To strengthen should I attached strapping underneath (cross diagonal) and see how that goes?

Or.

I could attach a small piece of hardwood the length through the middle (1.5 meters), and then get a large paver (x2), and then install an adjustable post (screw top) (x2). There is no way to dig a hole underneath without pulling up the deck. The problem is the paver could just sink and become useless until I get under and screw it up again each 6 months.

Thoughts?

Hi. 3 meters is excessive and I am guessing that they used 90x45 pine? Generally you would want to keep it at 2m max or 1.5m which would be better.

Some options, you could screw into the existing joist a hardwood joist that will make it rigid. You would only need to use batten screws every 30-60cm.

The other option is as you described, propping up the middle point and resting the base on a large paver. If it is large enough then this shouldn't sink that much but of course it depends on your soil. You would want to get a car jack under there to increase the height by 30-50mm to allow for settling.