Honestly, anywhere you feel comfortable. Some folks start chatting about proposals here or the OSM Discord or the OSM US Slack or one of the various smaller communities. Being specific about the kind of feedback you are looking for and questions you are still working through can really help. Necessarily, the conversation will need to expand to a broader audience as you move closer to a “final” proposal but you don’t need to start at the broadest possible place if that seems daunting.
As a recent example. Another mapper and I have often lamented to each other about a specific tagging deficiency, mostly in DMs on the OSM US Slack. That lead to this long thread that has a ton of great discussion and eventually landed at this proposal.