The Brazilian side of the park takes about 3 hours to walk along the paved path and take pictures. There is also a walkway out into the river.
The bird park, on the Brazilian side, takes about 3-4 hours too.
You could spend a whole day on the Argentine side, on the walkways over the tops of the falls, and cooling off in the river.
You can take a boat ride under Devil’s throat from either side (may be cheaper in Argentina).
There are helicopter rides. There is a visit to the dam. (neither of which I have done, so can’t comment.)
Going to Paraguay is a waste of time, IMO, unless you are enamored of cheap junk.
As to “reduce the length of long trip”, by bus I presume, you need to understand that these days in Brazil flights are often cheaper than the bus fare. Rio to Salvador by plane is about 2 hours.
For me, who also likes to linger a bit, rather than rush, your itinerary still has you spending too much time in most places mentioned. You could see a lot more of Brazil in 6 weeks without feeling rushed. I would suggest adding Salvador and some of the Colonial historic towns in Minas Gerais or the Recife/Olinda area. One or two of these would fit comfortably into your schedule.
Are you flying into Sao Paulo or Rio? If into S.P. go directly to Iguacu from there.
6 week sample itinerary:
If flying into Rio:
Rio de Janeiro (4-5 days)
Bus or flight to Sao Paulo and on to Iguacu
Flight and Puerto Iguazu Argentina (3-4 days)
If flying into Sao Paulo, go direct from there to Iguacu.
Flight to Sao Paulo, then bus to Paraty 1-2 days, then Ilha Grande 2-3 days,
back to Rio
(Forget Ilha Du Mel)
Or if flying into Sao Paulo, go to Rio now
Rio de Janeiro (4-5 days)
Fly to second destination (choose Salvadorarea and on to Recife/Olinda area—lots of beaches-- or Belo Horizonte for Colonial cities) If B.H., bus back to Rio. If Salvador-Recife, fly back to Rio.