Q.Brussel Sprouts wintered
I live in Michigan and we had, amazingly, a mild winter. When the snow cleared, I had a horde of brussel sprouts on tall thick stalks. It’s now the end of April and I need to know what to do with the plants that continued to grow throughout winter. The stalks are now covered with growing leaves and have a some small sprouts. Do I leave them alone and let them grow? Cut them back? Pull them out of the ground and make way for new plants? What’s the best course of action?

If your plants are producing sprouts, by all means allow them to grow.
You can always plant new plants in another location and then remove the old when you have finished harvesting.
Succession planting is a great way to utilize your garden space and increase the garden harvest.
https://www.gardeningknowhow.com/edible/vegetables/brussels-sprouts/pruning-brussels-sprouts.htm
https://www.gardeningknowhow.com/edible/vegetables/brussels-sprouts/how-to-grow-brussel-sprouts.htm
https://www.gardeningknowhow.com/edible/vegetables/brussels-sprouts/harvesting-brussel-sprouts.htm