I recently noticed that there is even a fcitx port for android (which runs smoothly with app support on SFOS when inputting in android applications).
Given that fcitx5 is very well developed and that the input method of SFOS seems to be having quite some trouble (SFOS 5.0 support no CJK till today, and lots of features missing for all languages), will it make sense to use fcitx directly? Or is there any special difficulty? (very different DBUS API?)
Can’t answer the good idea part, but looking at the compile instructions/dependencies it looks like it would at least be possible to compile it for Sailfish OS – if all the X11 stuff can really be disabled, and the old qt 5.6 from SFOS will make fcitx5-qt compile and the necessary protocols are supported in the custom Wayland compositor.
I believe SFOS’ predictive input still contains parts of the old Nokia system, which was very good; esp. Finnish is still better than whatever Google offers.