When ever I replace a heat pump with an attic indoor unit, I put in a
walkway using 5/8" to 3/4" plywood. Plywood less than 5/8" will not
support my fat ass. I go at least 18" wide and prefer 24" wide. The
walkway is required by code here, but not enforced well/ I figure it
into the price of the job, it is cheaper than replacin a ceiling my
foot went through. It is also better than having an employee hurt,
even though we have workers comp.

But it sounds to me like you have a problem wthh your trusses. Even if
you add flooring, your trusses will not get much stronger, likely your
problem will contimue. Perhaps you should twin your joists (add other
joists next to your existing joists) and fasten them together with
screws. Then put the flooring on top.

Stretch