If you take out the S1 (if you are entitled) you will be stating your intention to move to Spain permanently and will therefore no longer be entitled to National Healh Care in the UK on your return visits.Thanks for the welcome!
The honest answer is I'm not sure how long I'll be over. My initial thoughts are 6 months to a year, maybe longer, depending on circumstance so looks like an S1 form will be the answer!
An EHIC card will, as stated, cover you for emergency treatment only, therefore any normal illnesses would not necessarily be covered. Repatriation (should that be needed for any serious injury) will probably not be covered. You are playing a tough sport)
I would say coverage with Sanitas as mentioned above might be your best option