In linguistics this is called "code switching", and it is extremely common among native bilinguals.

Have a relative who has another language as their mother tongue but has perfect of my native language so we code switch a lot, I'm not fluent but I enjoy throwing in bits of what I know and they seem to appreciate it and encourage it

Weirdly, I kind of like parts of their language so I have lots of fun with wordplay and playing with the spelling and such and mashing them together for comedic effect like Sacha Baron Cohen does in Bruno

Yes, that is very normal for multilingual people. It's called code-switching and it has been intensively studied by linguists.

https://en.wikipedia.org/wiki/Code-switching

I'm bilingual English and Mandarin and I mostly do this in Chinese restaurants. The real hole in the wall places with the best Chinese food where the servers greet you in Mandarin by default if you look Chinese. Mandarin is more "computationally" expensive for my brain because I'm so used to speaking English so as soon as I have to express something complex I'll just blurt it out in English instead of stuttering it out in Mandarin, which prompts an English response from the server, and we'll go back and fourth switching between the two languages.

Yes, eng fr spa, mix all sorts of words together or use gendered words ungendered when using them in English

I've been noticing that when I read an English text to someone who also speaks my mother tongue, that I will switch to my mother tongue for reading out numbers. For some reason, it feels pretentious to pronounce it in English.

TurkoGerman here and we do that all the time. Our families back in turkey learned enough german by now thay we even do it in turkey...

Same for my tunisian wife (Arabic instead of Turkish though).

Quite common in India to speak a mix of English and another language. My partner and I mostly speak English, but some sentences just happen in Hindi.

I speak english, italian and arabic. and it's so much fun to switch between the three with other multi-linguals, personally sometimes i find it hard to switch to english after speaking italian for a long period of time, and when i read english text i tend to pronounce the numbers in italian as it feels much easier and makes more sense for me.

I think it's overall a fun experience.