I used to use the アルテ keyboard because of the "curve flick" shortcuts that made typing faster. I liked it, but I ended up switching to Gboard just for the ease of switching languages/keyboards while typing.
I use simeji keyboard. It seems to have a lot of japanese text and emoji shortcuts. Feels like made for the ecosystem of native japanese users. Use the "guided flick" japanese keyboard.
Microsoft swiftkey is nice, but the autocomplete is a bit different between english ime romanji keyboard vs the flick japanese keyboard.
