This is mostly true, but since you've worked on high power transmitters (I have as well, but on radar equipment for the Navy). Then you also know that the higher the voltage is, your skin presents lower resistance to it as the voltage increases (especially if it is wet).
I just don't want someone who knows next to nothing about electricity to read your post and think just because it's
[email protected] or 2.1A that it can't kill you, because it can. You should always respect the potential of electricity, no matter the voltage or amplitude.
De-rate your conductors, I wouldn't use thermostat wire, since it isn't for use in this type of application, like at all.
Stranded wire of the appropriate type is better for almost every application related to gardening because we move things around. Unless it is going to be installed somewhere permanently (like on a fixture) then sometimes solid conductors are better, if they aren't subject to any sort of strain and don't get moved frequently. Copper work hardens quickly and will crack over time if subjected to strain in the same place repeatedly.