Since you mentioned you only play on PC and mobile—and I don't know many PC games because I don't own one—I'll stick to mobile recommendations. If you haven't tried them yet, give Cybird's other games a shot, like Ikemen Prince, Sengoku and Villains (though I've only played one route in Villains, so I can't say if it's quite as good as the others, but the rest are definitely solid!). Another one worth trying is Lovebrush Chronicles; after Mystic Messenger, it's my favorite mobile title. It features four love interests and dedicated routes for the guys—and you can even get bad endings! If you prefer something more mature, you might like Love Unholyc, it's similar to Mystic Messenger in that you have to participate in chats at specific times rather than relying on tickets. However, getting the good endings is pretty tricky—at least when I tried playing, I couldn't complete a single good ending; I got frustrated and dropped it lol. If you want something sillier, there's My Horse Prince and Hatoful Boyfriend, they lean more towards comedy or parody than straight-up romance, but they're fun! There's also Magical Paws, it's really cute and short, but fun too! Oh, and Hakuoki and Amnesia: Memories are available on mobile as well; if you search around, you can download them for free online lol. I'm not sure if it's still available for download anywhere, but Liar! Uncover the Truth is a good game too!
Finally, if you don't mind using emulators on your phone, you could try Sweet Fuse (available for the PSP emulator). And here's one that isn't strictly an otome game but features romance along the journey: Persona 3 Portable! There's also Hakuoki for PSP, so if you want, you can play it on an emulator instead of mobile (though I actually think the mobile version is better because it has extra content not found in the PSP or even the Nintendo Switch versions haha). There's also the Nintendo DS emulator; on that one, you can play games like LoveRevo, Vampire Knight, Prince of Tennis, and the Tokimeki Memorial Girl's Side series (in my opinion, those are the best games for that emulator, and I highly recommend them!)