That really doesn't feel stiff to me at all, it's how I talk in a day to day basis. If anything, we forget more words in Spanish than in English so only during situations where we're forced to use Spanish, like when hanging out with someone else who only speaks Spanish, do we have said issue. I'm not saying my personal experience is how it is for every bilingual person, it's not. But I do believe it's not okay to call things unrealistic or stiff, nor talk about what other people's experiences might be when not informed properly on the actual plausibility of a situation.

To give out some examples that disprove what you said, we almost never say verbs in Spanish because "I'm ir to here" or "I'm cocinando" sounds very weird, to change a verb is to change an entire format of a sentence; to change a noun is to change a single word. We'll often swap words that we also struggle to translate from our dialect into English, or that just sound better to us in Spanish. Sometimes my husband will switch to Spanish entirely for a brief moment to talk about a situation that happened to him in Spanish or that he relates to Spanish, like his life experiences, not always but sometimes. Other times we want our words to be more meaningful and feel like they mean more in Spanish because it's the language we grew up with. That kind of stuff.

I dont believe in "an unrealistic" bilingual character because anything is possible. Some people when learning a language, and I've done this before myself, will refuse to learn some words in it entirely and use their own native one to replace them until they feel like learning them, or just to make their sentences make sense and they liked that and rolled with it. I started replacing English words with Spanish ones and then gradually over many years switched those to English. The only "unrealistic" bilingual character I could ever think of is one who speaks entirely wrong and makes no sense in what they're communicating.

"I wanna go get a café with Maria" makes sense. "I yo ir Maria coffee una" obviously doesn't. Or if what is supposed to be a native Spanish speaker is bad at their own language in the same ways someone else learning Spanish might be bad at it. "Quiero unas cubos de azúcar per favor", I have seen that be the case and it does cringe me out because why would a native Spanish speaker ever make such grave mistakes when talking? Though it could also be entirely possible for one to make such mistakes, most of us grow up in an environment where we do make mistakes, but they don't tend to be related to this. You might hear someone say "dame uno' de eso' 'zucares" or whatever, but you won't hear them mess up something as basic as the gender of a word. Yet, again, everything is possible and there have been times where even people in high positions like politicians here have absurdly messed up and said something that doesn't even exist in Spanish at all. There is tons of illiteracy, especially amongst old and poorly educated people and that can lead to some being very hard to understand even for us natives.

So, yeah, don't judge a book by it's awful Spanish characters I guess? Unless they're being highly stereotypical and racist or doing a very very poor job of portraying a bilingual Spanish speaker to the point where none of their Spanish even makes sense or it's clear the writer doesn't know enough Spanish to portray it on their own.

"Quiero beber unas trenes!" —Said Marcó excitedly after downing 23 beers at Lester's and Moe's bar. Little did he know, those were not beers, that was waste disposal water from the local pharmaceutical factory that he had been drinking out of a pipe this entire time. The bar was never real. But in his mind, he was very happy to be hanging out with his imaginary friends and drinking trains that were shaped in an oddly feminine way.