Welcome back, Gary.
This the same bathroom you been fixin' to work on for a few years and have a few other threads about? We like to get all the available info in one place whenever possible.
We'd need first to know what subflooring you'll end up with and its condition before advising what to install over/instead of/in addition to what you've got.
For sure you do not wanna add any 1/4" plywood of any kind to the package. If the subflooring needs some help, 3/8ths" is minimum and half-inch would be my preference.
If your subflooring is adequate (I doubt seriously it will be after removing the CBU) you could add thinner CBU if all you want is extra height and then your heating system. Far better to add more subflooring and install your heating system directly on that, presuming, of course, that the manufacturer approves that installation.
My opinion; worth price charged.