I have a page on inputting Japanese (not Chinese) which includes a section on OpenBSD.
https://srobb.net/jpninpt.html#OpenBSD. I did change
GTK_IM_MODULE=fcitx to GTK_IM_MODULE=xim
This worked for me in 7.1. I don't have a 7.2 install to play with. Generally, in Free and OpenBSD as well as LInux I use in .xinitrc
export XMODIFIERS=@im=fcitx
export GTK_IM_MODULE=fcitx
export QT_IM_MODULE=fcitx
#export LC_CTYPE=en_US.UTF-8
export LC_ALL=en_US.UTF-8
As mentioned above, I have, at times, had to change export GTK_IM_MODULE=fcitx to =xim.
That is a FreeBSD system using fcitx, not fcitx5. I have used it successfully in OpenBSD-7.1 with cwm and urxvt terminal. I did have to use nvi, available as a package, rather than OpenBSD's vi.
Again, note this is Japanese, not Chinese, but I did get it to work.