Indeed it is.
"Eu sempre quis" is the right one. It means I've always had this wish, it's like my dream.
But there are contexts where "eu sempre queria" fits:
Quando eu era pequeno, eu sempre queria brincar no lago. = When I was little, I always wanted to play at the lake. (It's not like a dream or a wish the boy wanted to fulfill, but it means the boy wanted to do it very frequently)
Os outros alunos sempre pediam para ela ensinar espanhol, mas eu sempre queria aprender português = The other students always asked/used to ask her to teach Spanish, but I always wanted to learn Portuguese. (Again, not a wish to fulfill, but a constant oposition against the other students)
Tell me if you agree with me.
"sempre" + "pretérito imperfeito do indicativo" = some looping action in the past
When I read: "Quando eu era pequeno, eu sempre queria brincar no lago", I feel it calls for a complement like "every summer" or "every opportunity".
At "Os outros alunos sempre pediam para ela ensinar espanhol, mas eu sempre queria aprender português", I feel it happened many times.
It is like if the adverb (sempre) changed the meaning of "pretérito imperfeito", which is a time interval in the past, to several time intervals in the past.
You got it right!!
I would say not several time intervals, but several times with intervals between. (Anyway.....I believe this varies from case to case, and your idea is very good)
Found a good translation: Eu sempre queria = I used to always want - or - I was always wanting.
The funny thing you made me see is that in this "sempre quis" sentence the "sempre" + "pretérito perfeito" changes it from a single ocurrence to a long term/time period occurrence. Very tricky, indeed.