Daniel, so exciting to be planning your Camino.
My son and his wife started bicycling the Camino in St. Jean Pied de Port (after cycling thru Italy and France). I met up with them in Pamplona September 8th last year, and we enjoyed 21 days of cycling (with a few rest days) to get to Santiago.
Your starting place will likely be dictated by the amount of time you have to spend on the Camino and the pace at which your group wants to ride. An easy formula to determine this is to estimate how many miles you wish to ride each day (we averaged 30/day), and multiply this by the number of days you have available for your camino (don't forget to add rest and sight-seeing days). Use this number to determine your mileage from Santiago, and this will help you decide where to start.
If you are trying to save time and eliminate the "more plain landscape", I would suggest skipping the Meseta between Burgos and Leon. Depending on your pace, that might only cut out 3 days or so. Attached is a photo of the typical scenery in this area. We utilized a taxi from Sahagun to Leon due to severe storm advisory. But there is also a train service that can accommodate bicycles along this route, though it only runs once per day.
I have no info to offer you on the northern route. Sorry.
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Buen Camino