As Saige composite decking installer you can use 4x2 treated joists as with all wood will rot eventually, if you i would put down mot type 1 wack it down then put a good quality landscape fabric down will reduce the rotting of the timber, you might have to dig out first of course. As for decking joists i use ryno metal joists as this will give you approx 25 years plus, may be expensive but at the same time will give you a piece of mind. Hope this helps.

As many have stated above. Use pressure treated C16 or preferably C24 graded par timber. With good circulation this will last a good 20 yr.

In regards to spacing w fit Trex and Ecoscapes and they both advise no more than 400mm centres on joist work (4x2 or 6x2 ). We fit st 350mm

Hope this helps.

Millboard make a great composite plas pro frame , which is rather expensive but will last over 20 years if installed properly. great for swimming pool surrounds ,and all wet damp areas , can be tricky to work with and it expands and contracts depending on the tempreture but is a great alternative to using timber
with regards to using a c24 timber frame , I prefer it and its easier to work with and will also last 20 years in installed properly and painted with a good solid coat of wood preserver before installing the decking boards

It makes sense to apply a wood preserver to the whole frame before fitting the deck boards it can be applied by garden sprayer as the frame is the structural part of the decking well worth the extra cost and will give you piece of mind. Composite frames are very good but pricey, Done right timber decking will last 25 years. Its fine for Trex or Millboard.

I prefer to use wood but make sure its pressure treated. The industries leading chemical is TANALITH E its a water based wood preservative that contains copper and proven organic biocides. As already stated, all wood will eventually rot but good airflow under the decking ensures you will gain maximum life from it and it can last upto 25 years. Each fresh cut needs resealing so use enselle on all fresh cuts. Its applied with a paint brush and retains the integrity of the pressure treatment. Check with the manufacturer for joist centres and always use a strucurally graded joist C16 or C25. These have been planed (not rough sawn) and have rounded edges. Make sure your frame is set out of level by 1 to 2 degrees to allow water run off.

All good answers but what I would say is if you see the fitter painting the framework for treatment. DON'T USE IT.
It's more or less the same price for pressure treated c16 grade timber.
Available at all timber merchants and commercial merchants.
I personally use timber for my framework as you create more options using it.
Composite framework and steel is so expensive it's scares away customers rather than reasurring them.
Traditional and easy all the way.

Best wishes.