I made the transitions between copper and plastic with standard brass
compression fittings rather than plastic pushfit stuff for robustness.

Never tried that. Always used pushfit for copper to plastic, and never
had a leak, even where I've cycled the same mains-pressure fitting
several times - though I can understand the feeling that those tiny
stainless steel teeth in a plastic compression don't seem an awful lot
of substance to hold it all together.