I feel the same way as you. My first language is English, and I have a good level of Spanish. The thing is, I definitely stutter a lot more in Spanish because, as you say, I have a lot less control and flexibility in that language. It is not so automatic for me to change my sentence in the middle of speaking, for example. It feels a bit like what I was like as a teenager - I of course knew English but I didn't quite have the gymnastic power that I have with the language now. It's disheartening because it took me over 20 years to sound passably fluent in English. It's also difficult for me to assess my Spanish level because my lack of fluency is definitely a combination of lack of proficiency and my stutter. I guess that, like with English, I will only be properly fluent in Spanish when I have a very high degree of versatility in the language. I would like to do the C1 Spanish exam but I don't know how much stuttering would impede my ability to pass the speaking. I've checked the DELE criteria for accommodations but it doesn't mention speech disorders.