As you mentioned, for developmentally appropriate mini-lessons, the time should be kept to about 6-8 minutes for your first graders. Try to map out when your kiddos are being pulled, and see when you can find that much time when you have them all together. It may not be the perfect time, but hopefully, you can find those precious times throughout your block. You may have to have a round of Read to Self or Writing before you have a mini-lesson--not ideal, but you can make it work.
As far as reading groups go, I suggest you think about switching to one on one conferring rather than guided groups. I think this will be less stressful for you, and also that you'll find it more appropriate for your students. There are a lot of articles/videos here on the site to look at that.
Here's a link to a short article by Joan about switching to conferring : https://www.thedailycafe.com/articles/moving-from-mostly-small-groups-to-mostly-one-on-one-conferring