The tab button moves the indentation of the entire text block, if its the first character in a block of text.

I'm writing a fictional novel in Google Docs and copied it over to Proton Docs to give it a try. Immediately dropped it because I can't indent the first line of a new paragraph unless I press the space bar a bunch of times. Or press space once, then Tab.

Come on now. The overwhelming majority of people are going to use the tab button for first line indentations. The only way I see it being useful as is, is if I was scripting Python in a word processor for some reason.