I'd rather have other options for variable rates than FRR. Limiting FRR arbitrarily with "magic" values (100k USD? 500k USD? 50k?) is not really useful imho.
This is basically what I would suggest, the problem is not a variable rate, it is that there is only ONE variable rate.
I don't think it will be a very complex UI change, and should not increase the difficulty very much. What I imagine is something like
FRR + x
Where x is a user specified value. At first, if everyone leaves it as the default value of 0, we have an identical system as we have today, but over time, as some people use negative values and others use positive values, the offers should start to scatter around the FRR.