I pass through Bilbao or Santander several times a year, at different times, because I use the ferry to/from Portsmouth/Plymouth.
With the proviso that I'm at the start or end of a 1,000+ km drive while I'm there and don't tend to hang around, July can be both chilly and wet. It's not Costa weather and is extremely unpredictable and unreliable. In July it is not unusual to be plagued by clouds rolling in from the Bay of Biscay and the clouds are often trapped by the mountains immediately to the south. You'll hit mid twenties at sea level, much more in the sun, but it's not like the south of France by any stretch of the imagination. It seems to rain permanently there.
If you head inland to join the A1 from Vitoria-Gasteiz to Burgos you're only trading cloud cover for altitude, which doesn't help the temperatures, and in the last 12 months the high altitude plains have also been pummelled by unseasonal rain. I've seen it fall from 18 to 12 degrees centigrade in Valladolid at noon, in the last week of June 2024, in a thunderstorm.
In Bilbao itself you will find Castilian Spanish speakers no problem, and they do speak well for a student to practise, but a fair bit of Basque too (hint - if you use a voice-activated petrol pump DO NOT choose Basque!). Previous posters have suggested NW Spain - A Coruña and Vigo - but in my experience these are pretty much the parts of Spain where you're least likely to encounter Castillian Spanish (in favour of Galician). I have always found it much more productive to speak Portuguese rather than Spanish there.
Others have also suggested the Costa de la Luz. Thirty years ago the temperature there almost never exceeded 40 degrees for more than a couple of hours. Now, it is not unusual for western Andalucia to go for several weeks at a time in excess of that.
Honestly, you'd probably have a great time in Bilbao or Santander but do not rely on the weather. If you can handle public transport that's great, but you can take the ferry and have your own car there.