Google Document to BBCode converter · 8:55pm Jun 5th, 2015
As much as as it is converting a large GDoc to bbcode so they work on fimfiction, a little looking around found an add-on for google documents that converts the document into BBCode. You can find it here.
Images have to be inserted manually, but it's still a substantial improvement. BBcode fimfiction doesn't use like lists obviously won't work, and anything with a size tag should be updated to fimfiction's units of "#em" (don't forget that it's a scale here, too).
I always just wrote it with the BBCode in. It's not difficult for me to remember.
I'm also a mix of English nut and computer programmer, so I'd imagine such things would be a constant concern of mine. Syntax is very important to be consistent in and even a single missed mistake can break everything, so I can't say my way is the easy way.