I've looked deeply into creating language pack for wikitext and I have some conclusions. Lezer grammar is impossible to be written for wikitext thus resulting in necessity of writing completely custom parser (like is done for Markdown). This is a lot of work and unfortunately cannot be done alone. I recommend to fully focus on stream-parser way for now.
From my observations downsides of using stream-parser are with nested styling (maybe not a case here) as e.g. in Fandom we had headings that have equal signs in blue color and this heading was having bigger font size, I couldn't achieve having that bigger font size without having text between equal signs in blue color instead of white.