I have a command palette tool which can also be used to select instruments quite nicely imo.
There is only one shortcut you need to set to open the palette, then press i and type a number, which can also be multiple digits long. Alternatively, if you have a lot of instruments and don’t know the index you can press Enter after i and get a search field where you can narrow down your target by typing fragments of its name.
The same kind of thing works for many other things like tracks or dsp devices, but instead of i you press t or d and so on.