Can you do this? I'm making a lot of flashcards in Anki (Kanji in Context workbook exercises) and it slows me down switching from romaji and kana to get the brackets in there for the furigana.

No, as far as I'm aware, [] are only on the romaji side of the J-keyboard, not on the kana side. Because of that took me awhile to find them the first time.

The Google Japanese Input lets you do this without switching back.

Copy the brackets to your clipboard like this [], paste them with a quick press of ctr+v and then press left arrow.

I unfortunately can't help you with your original question. However, it might also be useful for you to know that there are already full spreadsheets/anki decks/etc. (even including audio for just about everything!!) for KiC on this forum. If you want to save yourself the trouble of re-typing everything (assuming you're not doing it for pedagogical reasons), shoot me a PM/email or search old threads on this forum.

What operating system?

As tokyostyle says, Google Japanese IME knows about regular [], but if you specify your OS maybe another solution can be found.

I'm using the Mac OS. I sometimes boot under windows into boot camp but mostly I'm in Mac OS. No I'm not entering stuff for the pleasure of doing it myself or because entering it yourself is a wonderful way to learn it in the first place - I think it's painful and therefore bad for learning since it produces burnout more often than not. I actually found a thread for KiC here though and sent my picture of my books to download. It's a nice spreadsheet but I noticed they either used an older version of the book with fewer sentences or skipped the ones they didn't want to enter. There's a lot of skipped ones. I'm flip flopping back and forth between doing it myself. I think I will just use the file with the missing sentences because I'm entering 試飲完全マスター文法 anyway. I looked for that one too but it's not done the way I want it and I doubt there's one that is. I think if I'm going to learn the grammar I really should do it myself. (Unlike vocabulary I think making the cards actually does help)

On Mac, hit option+[ while in Japanese input

thanks! that works perfectly. Is there a way to do the romaji space on mac in kana input as well? The furigana add on for anki needs a romaji space before the word you're putting furigana over.

A romaji space would be shift+spacebar

https://ankiweb.net/shared/info/1627628203
Use that add-on, it'll be way faster. Use it to automatically generate furigana and simply change them when the generated reading is wrong. That way you (almost) won't have to type anything.