For example, on Android smartphone / tablet, I am using two Japanese IME’s at the moment - Google Keyboard’s QWERTY Romaji IME, and handwriting (手書き) IME.
手書き appears to be very convenient. as it recognizes even small Kana and obsolete Kana. I don’t have to scroll, and find things like Katakana that IME doesn’t like. Even more is the case for Kanji compounds.
Now, for laptops, are there any good options? Are there any convenient tools I should buy? (e.g. I might be able to write on a tablet input device, but it won’t be converted to searchable text. But then, I haven’t bought a tablet device yet.)
Actually, my laptop already uses a touchscreen, and comes with a stylus.
I am 100% certain Windows can, and by Microsoft itself, and actually, I might have seen it; because Windows support touchscreen much more since Windows 8.
Linux might need a dedicated software, but it’s not the default (perhaps CellWriter, but not sure if it works as an IME) - tegaki-recognize seems to be the best one, but nowhere as good a Google Keyboard for Android. Also, for some reason, it doesn’t output Katakana.
No idea about macOS, even if you can plug-in a tablet input.