Cat I approaches are by definition by sole reference to Baro altitude.
Not true, actually. There are Cat I ILS approaches for which Rad. Alt. DH minima are calculated, surveyed and quoted. My operator's manuals include the comment "...Radio Heights, where provided, should be employed for all autopilot coupled approaches."
The crucial point is, of course,
where provided - because of terrain variations (as you quite rightly said) not all approaches will have terrain which makes a RA DH possible for a 200'-ish decision point. But many are, and it is incorrect to imply that a Cat I approach is limited to a Baro decision.
LHR 27R is an example - my performance manual quotes minima of 200'
Radio Height and 550m RVR explicitly for a
Cat 1 coupled approach.
BRGDS