First of all if you dont like Maths then choosing not to take it is absolutely fine! Prioritising subjects you like apparently makes your life at a level a lot easier!
I dont have personal experience of Spanish a level but its a good one if you enjoy it and are able to learn the content quickly. I think
@emm4nuella does or did a level Spanish (sorry if Ive got that wrong but that’s the only person I could think of that I think I saw mentioned it) so maybe they could give you some insight into the subject, if they have time (I feel so awkward tagging people but feel bad that I dont know enough about Spanish a level to give a proper view).
Spanish will probably give you a nice great from the STEM stuff and could prove very useful in the future if you would like to work outside of the UK so I’d say its a good combination if you like all your subjects!
Hope this helps and you get some input on what’s involved in the essay stuff at a level, I would say that as you really enjoy it though its a great choice and a great skill to learn a language at that level. Have a great day