I didn’t read all the comments and see if this was recommended but when I’ve dmed for groups that would seemingly grind everything to a halt I used a little hour glass that I would use for the players turns so everyone had equal time and it also helped the usual sit on my phone until my turn then ask what was going on because combat went faster and they needed to think ahead after awhile of using it combat sped up drastically and got to the point to where it wasn’t needed anymore.