I was surprised / confused to find multiple instances in the dock too. I adjusted the setting, this stopped it happening and I seem to remember doing it here (not 100% but another option to try):
System Settings > Desktop & Dock > Show recent applications in dock
Take a look at Ujam Finisher Retro if you're looking for something similar to ART: https://www.ujam.com/finisher/retro/ If you want more control, then you might want to try multi fx plugins like XLN Audio RC-20, Excite Audio Lifeline Expanse and Console, and Thenatan Vybz. This looks great. Thanks ...