I have a degoogled phone (GrapheneOS), and therefore, the slot that is usually taken up by Google Assistant (or whatever "AI"/ voice control your manufacturer uses) is free.
On Android, you can manually select what tool you want to activate when swiping from the edge up or holding the power button. At least on my device.
Right now, it's only Firefox (search) and Kvaesitso, my home screen, but not much more.
This means, that this system is very modular, and there can be quite a few apps around there that also have this feature!
Currently, I've set Kvaesitso as "assistant", so when I swipe up from the edge, the search opens up instantly. This enables me to instantly search and open apps, perform basic calculations, search the web, and some more. Sounds neat, but I can also achieve that by just swiping down on my home screen.
This however feels unnecessary, and I'd like to replace it by something more useful.
It doesn't need to be an alternative to Siri or whatever voice assistants you think of, it can be whatever you think is useful.
For example, a search bar, some quick access, cool tool, whatever.
I've already heard of Mycroft, but I think I don't need that tbh.
I'm also intrigued by the smart AI picker (circle whatever you see on your screen, and it tells you what it is) Google introduced, but I never tried that to begin with. It just looked cool in the ads, but that's their purpose I suppose.
My only limitation is that it's FOSS. Sure, I could just download Google Assistant or whatever myself, but I don't want to defy the progress I made with all the privacy and security implications. Those assistants often need quite a few privileges, and I don't want to give some proprietary app my screen information for example.
I think Dicio is probably what you are looking for.
If you are a Home Assistant user you can set up an assistant and set the Home Assistant app as your Android assistant, though this is substantially less useful if you don't use Home Assistant.
The company behind Mycroft got bankrupted by patent trolls (and perhaps poor management), I don't know if there is any development there any more.
Home Assistant is a great platform to build on and their assist feature is amazing. Definitely worth the effort.
I have recently got one of their Voice Preview's and it's awesome, but I'm using cloud features (including ChatGPT backend). I think it depends on what specifically OP is looking for in an assistant as it's not going to give you directions, search the internet, etc.