My usual suggestion for an easy-to-read author is Akagawa Jirou. He’s written an awful lot (hundreds of books) including in the action kind of genre. Generally they’re set in the present-day Japan of when they were written (so the older stuff is an 80s/90s sort of thing). I dunno if any of them quite get down to less than 200 pages (other than the ones aimed at younger readers), but less than 250 is common. 赤いこうもり傘 (to pick one off my shelf that I recall as being action-ish) is about 225 pages; back-of-book blurb describes it as ‘humour/suspense’. His writing is straightforward, doesn’t generally use difficult vocabulary, and there’s a lot of dialogue, so 250 pages doesn’t mean 250 pages all full of characters I would also recommend ミス as a good action/humour one by him that I read pretty early in my studies, though it’s a bit longer at 330 pages. Or just look through amazon blurbs to see if one catches your fancy.
If you like detective fiction, Matsumoto Seicho’s 点と線 is good in the police procedural/how-did-he-do-it vein, and only 230 pages, but as a book from the late 50s it’s set in the Japan of its time, and it’s definitely not such an easy read as Akagawa.
Yoshimoto Banana is also an easy read but way out of your genre preference (tends to be stories about a somewhat unconventional young woman sorting her life out and possibly finding love).