Welcome to the group.
Regarding your garden railway _____
Just a few thoughts from some-one who has yet to build a garden railway but who has thought a lot about it.
In your place, with the amount of space available, I would probably build the railway up from the ground.
Either using a 'conventional' baseboard (using exterior grade materials of course) or alternatively a great big pile of rocks and earth.
This would bring the trains up to a better height to view them and also to work on them.
As you plan to use DCC I would suggest that you use a main bus under/near the track with feeders to each length of rail.
Do NOT rely on the fish-plates for electrical continuity ____ they WILL corrode.
It is also recommended 'best practice' for DCC indoors ___ so it should defiantly be followed outside.
Remember that you will have to keep the track clean so no long or curved tunnels.
I do believe that Lenz gold decoders, although expensive, have less problems than most others over dirty track.
No experience with them ____ just hearsay.
Although I have seen a loco drive from a length of track onto a sheet of paper placed over the track under complete control. Impressive.
As an alternative have you thought of using battery power with radio control
No power pick-up problems and no rail-cleaning
Just some of the ways to go.
Hope that its helped not hindered,