Treated Timber Base & Joist Fixings?

[plasteredric]

I'm planning on building a garden room, and going round in circles looking into what fixings I should be using for the base.
The plan was to screw through the beams into the ends of the joists and then use joist hangers nailed on for good measure.
When I've looked into it, some websites say use galvanised nails and others say galvanised coatings react with the copper in the wood treatment and cause them to break down. Which leaves stainless fixings, but these are really expensive.
This also leads me to consider that the joist hangers themselves are galvanised, so am I to use stainless hangers as well? I'll be needing about 100 and at £5 ish each that's going to add up.
Watching the garden room videos on youtube they seem to just use zinc coated screws, but what do they care if they're all rusted in the 3 years.
Has anyone done something similar? or have any suggestions?

[Grendel]

My background in restoration saw the use of a lot of stainless steel , mostly A2 but sometimes A4 marine grade. It was practically our standard go to and I've used plates and brackets anything up to 20mm thick. It's a certain fact that the stainless will be around even after the timber has rotted away and I've removed perfectly good nails and screws in that way . I was always taught to use at least galvanised with treated timber as uncoated steel will rust away quickly. It was not uncommon to find old fixings that had been in place decades if not longer but one thing to remember is that they are largely wrought iron and so less prone to the rusting/rotting away that steel does. Stainless fixings with galvanised hangers should be fine , the nails or screws seem to rust quicker than the hangers if going down the completely galvanised route. You could always give the hangers a coat of rust preventative paint if you want . Rust seems to form more readily when in contact with the timber.
An alternative you haven't mentioned is perhaps to actually joint the timbers together . Maybe a little more time consuming but you wouldn't need to buy or worry about joist hangers...