Am eagerly anticipating the commercial availability of Mathpad.
At the moment I'm using Espanso, an open source software that lets users map typed character sequences to unicode. So it's possible to set things up in such a way that typing the character sequence ";" "a" ";" makes Espanso replace the entire ;a; string with the greek symbol alpha α.
Symbols like ⇒ that can kind of be "drawn" with common keyboard characters "=" ">" is possibly nice to be mapped to the character sequence ;=>; This is a personal preference inspired by Typst's math notation design choice.
I prefer using a Compose key [0] (remapped Caps Lock, for me), which can be installed/configured on all major platforms. Configuration is sharable via XCompose configuration files.
I like Vim's digraphs, which go in a similar direction. For instance, Ctrl-K = > gives ⇒, Ctrl-K a * gives α. An overview of available digraphs is available at :digraphs.
Otherwise I like the Agda input method of Emacs, where \to gives ⇒ and \alpha (or \Ga) gives α.
Somewhat predictably zsh also has a digraphs¹ feature, via the insert-composed-char function². Supported characters can be seen in source³, and beyond that there is the insert-unicode-char² function for when you need it.
I flit between regular compose key input and zsh/vim digraphs in a way that makes no sense to me whatsoever. Compose ^1, AltGr+1 or C-k 1S all kind of feel natural to me, but the advantage of the ZLE method is that you can also use it to preview characters which can be useful if you want to test something out while in another widget or find the hex value to insert using some other tool.
¹ https://datatracker.ietf.org/doc/html/rfc1345#section-2.3
² https://zsh.sourceforge.io/Doc/Release/User-Contributions.ht...
³ https://github.com/zsh-users/zsh/blob/master/Functions/Zle/d...