If I summarize all the basic math:
1) most of the droppers does not have offset => saddle moves forward even a bit more
2) The main diff for the frame makes seat-angle and Saddle-BB distance is ~8cm diff from old geometry => it is the ~same diff like Road and TT bike saddle
^^^ all the things are the ~same on M and L frames, so the only important diff is Reach
I do not care about stack in terms of saddle-height - it is just 1.5cm diff. But in terms of handle-bar height, sometimes lower stack helps to lower the handlebar without slam it down too much.
And looks like the main diff is really only top-tube length, which is 61 or 64cm, and to compensate your saddle-handlebar distance all you need is stem which is 3cm longer or shorter. So, frame M+10cm stem = L+7cm stem or M+6cm = L+3cm. Looks simple - all I need is to decide stem length.
Small point that L frame adds 0.5cm _back_ to the saddle => to hold the same top-tube you should substract the 0.5cm from stem too.
If I go M => it is not very aggressive position compared to L, I can compensate it with stem, but, probably it is not idea on descend. and 3cm shorter wheelbase, which, probably makes it a bit more nimble, which is a plus for the long bike
If I go L => stem is shorter, and body position is a bit lower too, the only negative thing that stack increased, which prevents you from going lower with handlebar
