Have you got that the right way around, Agilla de Campo is after Estella?
Agilla de Campoo in 17 days should be on. There’s a lot of climbing once you’re over the border pretty much whichever way you go but it’s 40kms a day-ish, I reckon I could walk it in the time you have, it was about 9 days walk from St Jean de Luz to Santander on el Camino del Norte.
You then go back to Estella which is odd. The easiest route will be down to Burgos and then reverse along the Camino Francès bike route – perhaps 260km of hilly but not mountainous terrain in three days, possible.
I can’t remember much about Estella apart from the free wine tap for pilgrims at the local wine co-op.
Edit: a word of warning, once you start walking along el Camino it’s very hard to stop, and whatever else you had planned may suddenly seem inconsequential so you just keep walking. 😉