I am unsure that I have understood this correctly - if you are deepening the rebate for a wider/taller door, then the following applies (and yea you would need to remove the architraves, which presumably you would need to anyway):
This is not dissimilar to a job I have ongoing - this involves reducing the underside of some intermediate cills in some tall windows a bit complicated to explain, but essentially the same job as the OP has to do on the head of each frame.
The approach I took was to set up a straight edge at the depth required - I used a wide life of plywood to give a stable base for the router to run on - I added a wide base to the router for additional stability - I used this very expensive and impressive router cutter Whiteside Model UDP9112 Ultimate Spiral Pattern Bit 1/2 SH x 7/8 D x 1-1/8 CL by Whiteside
https://amzn.eu/d/61QCYHU - you can use this in stages of course, running against the freshly cut face. You would only have the router bit radius to clean up in the corners but the width of the router base at the bottoms - still about 80mm, I guess. [incidentally, I bought this bit used ages ago quite cheaply it now seems - I winced every time it nicked a screw tip in the workpiece!]
For the awkward bit at the bottom, I would try to rig up little platform to run a small cordless router over - I have been using this 12v Bosch which is surprisingly good
GKF 12V-8 Cordless Palm Router | Bosch Professional - the offset on this is 40mm, less half the router cutter width.
If I (and others above) have misread the problem, and you are deepening the rebate for a thicker door, the problem seems easier - but the little cordless router would be useful here too.
In any event what you cannot get to with a router cutter is down to a combination of multi tool, chisel and possibly little shoulder plane etc. On that note, for the awkward bits on my job I mentioned above, I bought one of these Shop Fox D3751 Precision Shoulder Plane
Amazon.co.uk - it took a bit of fettling, but has been very handy.
Cheers
