Fish can just be a shell. I only write bash scripts.
That's the exact problem. You deal with two languages. I did that too BTW. Now for someone who is experienced, its fine. But for someone who just started learning about Linux and scripting, its not ideal, as it adds unnecessary complications and friction for a learning process that is already complicated. Because learning Bash is a must do in my opinion. Regardless what custom shell you use.
After that learning process and getting familiar, one still can decide to use Fish as the interactive shell. But that's AFTER the initial learning process of the basics of Linux.