I HATE WORKING ON SMPS's ... I've done this a lot of unsafe, and semi unsafe ways in the past. ... and if you know what you are doing and understand your equipment limits you won't blow up your scope or kill yourself.... if you don't know what you are doing, or get it wrong, you can blow up your scope and/or end up dead.
Invest in an isolation transformer. isolating your high voltage equipment from a return path that can kill you us just basic safety, regardless of what ground isolation you think you can leverage to probe. Safety first!
The advice id go back and give my younger self would be to
get a proper high voltage differential probe... and that is the same advice I will give to you.
they may seem expensive.. but.. consider the alternative costs.
cost of a new scope and hospital or funeral expenses... seems like a bargain

though to be fair you probably would not shell out for a new scope if your dead...its likely and either or deal.
just so happens eevblog shop sells one at a reasonable cost.
https://www.eevblog.com/product/hvp70/460au
the minimum setup I use today is a differential probe, a variac, an isolation transformer, a load limiting setup (typically an incandescent light bulb that can be switched in series between the power source and the device under test - also referred to as the dim bulb test) if I don't have that full setup available, id rather not work on it.
watch these videos if you are unfamiliar with the potential issues and methods.
last edit: misspellings