I'm considering a vertical radiator with plug-in base loading
coils.

Which plugs and sockets are recommended, for the standard 4mm
banana plugs seem a little flimsy if the coils are made with,
say, microbore copper tubing?

Something akin to a banana plug but about 0.5" diameter
would seem to be apt

Currently, I have a 2" aluminium pole which is a nominal
1/4 wave for 20m, and a counterpoise of 100' running the
whole length of the garden which I'll tap in the middle.

TIA