Game on!

[Muzzer]

You may be overthinking or misunderstanding this. Having spent most of my career designing and developing mains powered SMPS products, I would endorse the use of an isolation transformer. It's true that the RCD no longer protects you against accidental contact with an HV circuit - but when it's floating, by definition one such point of contact isn't in itself an issue. On the other hand, if it's non-isolated mains, you can get a lethal (deadly) belt just by touching a live node unless you are fully enclosed in a heavy duty rubber gimp suit. Unfortunately these are expensive, uncomfortable and can get you arrested.

Most professional labs use an mains isolation transformer and a variable source, either a variac (variable autotransformer) or a modern switchmode AC source. There is generally an e-stop to completely kill the power remotely in case you observe anything untoward. And you don't let anyone near the stuff without full and formal training.

There's no protecting against complete idiocy and at some point you need to learn the importance of NOT touching 2 different nodes, particularly with different hands. I once did that across 415Vac and somehow live to tell the tale. It hurt like f*ck and I never did it again but might not have been so lucky. For that reason, only using one hand to attach probes etc, combined with an isolation transformer is an <almost> foolproof way to avoid unnecessary bouts of death. Nowadays, we are trained in CPR and have access to defibrillators but those are the ambulance at the bottom of the cliff rather than the fence at the top.