If you’re going for quick level-up time, there is also the challenge of doing radicals perfectly, but you indeed only start running the risk of kanji errors slowing you down when all radicals are Guru.
While I’m unsure if it’s still a 100% accurate, or if they tweaked some things, in the WK Guide there is a chart that shows the SRS intervals if you scroll down to the section Hold on, I have to WAIT?
Correct - vocab accuracy has no effect on level-up.
The system divvies it in hour blocks. If my level-critical reviews are unlocked at 09:00 in the morning, I can finish them at 09:59 without incurring any delays. The hour that they were unlocked in matters (that’s why a 4 hour interval can feel like three, if you do a lesson at the end of the hour).
If you use WK on computer, I find the userscript Dashboard Progress Plus exceedingly useful. It will show upcoming reviews, and any current level-critical ones are indicated by a small arrow, so you know exactly where you’re at.
Note that this userscript requires Open Framework to work, but all the instructions on how to get it running are on the script page. 