> use both a chain lock as well as a U lock that're both thick enough
No, thickness is an irrelevant property to an angle grinder. You're adding something like a second of grinding per kg of material. Makes no sense. The trick is to use grinder-resistant locks. Those extend grinding time to minutes.
Even those are variable in quality. Do you have a lock that takes 2 grinding discs to cut through or one that takes 26 grinding discs to cut through?
https://thebestbikelock.com/security/angle-grinder-proof-bik...
Or a tough chain slack enough that it’s hard to press the grinder against.