I wasn't talking about actual exposure lock, but about locking the brightness of the EVF when in exposure simulation off mode..By the way, I use *H too. It's a pity it doesn't work in conjunction with AE lock with the shutterbutton though.I think you can do exactly that. In addition to AE Lock, the R5 and many older dSLR's offer "AE Lock with Hold." You don't have to keep mashing AE Lock while composing. VERY handy when using Av.
You can access the option under the camera's button customization menu. The symbol is *H. It is mentioned VERY briefly in the EOS R5 AUG on page 826. Details are offered by Canon at this link:
Personally, I've transitioned to Manual mode for shooting situations where I need to hold Exposure (say when background light changes, but subject's lighting changes very little), but I still like Av for situations where my lighting on the subject changes rapidly. I mostly use Spot Metering, but the apparent exposure in the EVF plus the Histogram in the EVF have removed all the guess work that sometimes failed me on the 5DIV.