surface/external edge coupled microstrip vs (internal) asymmetric edge coupled microstrip
the calculations for internal trace impedance is less precise than for external traces, which is why they are often ignored.  I've built boards like this before for EMC reasons (in radios) so it is workable.  but the strip-lines are best on the surface with GND underneath) since the matched pair will couple to each other.
The other thing to do is talk to the PCB manufacturer, often they can tell you the trace width to use for a particular stackup.  With the frequencies you are talking about, it would be a good starting point as they might recommend certain prepregs to make it easier.
see also https://www.eeweb.com/tools/edge-coupled-stripline-impedance/