Thirding (Fourth-ing?) the fact that plain form gets used even in polite speech.
In a lot of longer sentences, the content in the middle is going to be plain form and the actual “politeness” です・ます won’t come up until the end of the sentence (assuming you aren’t using keigo or anything that results in literally different word choice).
Like, for example, if I want to talk about “the person reading a book,” that’ll be something like 「本を読んでいる人。」But if my sentence is longer, like “Oh, the person reading a book over there might be someone I know,” suddenly that’s gonna be in the middle of the sentence, along the lines of 「あら、あそこで本を読んでいる人は知り合いかもしれません。」
You still have the long ます for かもしれません on there for politeness (although tbh if I’m talking to one of my coworkers I would absolutely just be using かも here) but you need to keep/know the plain form 読んでいる in the middle.
And honestly, when I first studied abroad here in college I actually had a bit of a hard time because soooooo much of what we’d done in class had been purely です・ます form, and suddenly the students around me were using plain form way more than I was used to.
I think there’s also a slight habit, among Japanese learners, of being So Aware That Formality Is A Thing that it can be surprising how quickly and…well, casually casual Japanese gets used. I’m sure it’s impacted by a lot of the folks I interact with being fully aware that I’m not fluent, but I don’t stress too much about です・ます with coworkers and it’s never really cropped up as an issue for anything.
And finally like @cafelatte said, there’s also plenty of in-between levels. You’ll hear people use plain form conjugation and tack on か, and it’s not formal but it’s not casual or rude either.
TLDR: Plain form gets used a lot more than you’d think for both grammar and casual-speech purposes, so imo I think it’s important to practice. (But a setting to swap between depending on which one you need more practice for could be useful!)