We spent an extra day in Sahagún in Fall 2021. It wasn't a matter of planning, but there was in mixup in the date we were meeting another pilgrim there. We thoroughly enjoyed the extra time, and there was a surprising amount of things to do and see. The museum connected with the Cathedral in Burgos was having a multi-city exhibition of religious art, which included the church at the top of the hill in Sahagún, which was beautiful. I think that was at the Iglesia de al Virgen Peregrina. I don't know if there is anything comparably interesting there now. We also hit the town on a market day, and saw the amazing assortments of olives, dried fruit, etc. We went to the Pilgrim Mass in the Monastery albergue, even though we weren't staying there - a lovely, inclusive service. There was a lot of street art and displays of poetry (including, interestingly, by Emily Dickenson) around the city. Food was quite good, and the general history of the area as a significant crossroads was interesting.
My thought is that you can find interesting things to do almost anywhere on the Camino Francés, and sometimes it's fun to explore areas that aren't in the top 10 tourist places. It also looks like you've done the Camino Francés previously, which might make spending extra time "off the beaten track" more appealing. We were happy to have gotten to know Sahagún a bit. Would we spend an extra day in Sahagún if we were ever there again? No, since we saw quite a bit in 2021. But we would gladly stay an extra day in a comparable place.