Transdroid works pretty well. It's not the most up-to-date app, but it gets the job done. It supports multiple torrent applications including qBittorrent and it's fully open source.
Second this, it's my daily driver on android to control my qBittorrent instance, and so far I haven't missed any kind of functionality, it just does everything I need.
For basic tasks (uploading a torrent, changing download path, etc) I use qBittorrent Manager and it's pretty good.
Maybe this one? https://apt.izzysoft.de/fdroid/index/apk/dev.bartuzen.qbitcontroller