I am a British citizen living in Thailand.

 

Next year I am considering spending up to 5 months in Spain.

 

This takes me outside the 90 days in 180 days rule; what are my options?

I never understood this stupid rule; EU is loosing billions of wealthy tourist money every year!

 

Get a "long stay" visa to certain EU countries, like France, who like you.

 

"For any stay in France exceeding 90 days, you are required to apply in advance for a long-stay visa. In this instance your nationality does not exempt you from requirements.

Whatever the duration of your planned stay, the duration of your long-stay visa must be between three months and one year. In order to extend your stay beyond the period of validity of your visa, you must apply for a residence permit at a prefecture.

During its period of validity, the long-stay visa is equivalent to a Schengen visa, enabling you to move around and stay in the Schengen Area outside France for periods not exceeding 90 days over any period of 180 consecutive days, under the same conditions as if you held a Schengen visa.

https://france-visas.gouv.fr/en/web/france-visas/long-stay-visa

 

This is a little gray area and you must "shop around".

Or, one way is to re-do Brexit... or find a lady and marry her or become her "partner". This is very cheap and easy process in Spain!

Good luck!