I've completed 400km of the
Camino Frances over the past 15 months and need to walk the stretches from Burgos to Sarria (I walked from Sarria to Santiago in June 2019) to complete my Camino.
With all due respect to yourself and your questions, your arrival in Compostela did complete your
Camino.
The Way of Saint James is a Pilgrimage to the tomb of the Apostle in the Cathedral at Santiago de Compostela, regardless of anyone's religious beliefs or lack thereof. The latter are incidental to the
Camino, but the former is its essential. (which I know sounds like a paradox, but the foot pilgrimage to Compostela is the most unusual of all the Catholic pilgrimages in that the journey towards the Shrine has more importance than one's reasons for undertaking it in the first place ; which is a Mystery of the Way of Saint James)
It isn't actually a hiking trail, despite
every appearance to the contrary.
-- Having said that, there's no reason why those with time limitations and wanting nevertheless to experience the "full" length of the most popular version of the walking pilgrimage shouldn't do so, and so in regard to your questions, I'd say :
Walk for however much time you have between Burgos and wherever within your next two-week timeframe. Astorga ?
And so on -- but I'd really advise, don't just aim for Sarria ; this is a new and
different Camino, and you should aim for Santiago.
If you don't, it will feel incomplete.