Toilet flushes slowly, water comes all the way up, then water goes low, shower drains slow, but sink is fine
I would be running a snake down the main line from the toilet, depending on the plumbing the sink could be downstream from the plug. Normally the toilet location is where the vent connects so if it backs up the vent line can be flooded . Running from the toilet is usually the easiest access and only cost a wax seal.  A plunger knocking it loose sounds like a better case for a big snake so you can pull the clog out instead of have it move it around.