I'm so glad you brought this up here. You've received fantastic advice!
Whether or not you choose to submit to agents or editors, finding great critique partners will be a huge help! Even after years of writing, revising, participating in the Blueboard and conferences/workshops, I'm amazed at how many things my critique partners point out that help take my work up several levels, explore angles I never considered, and make sure it's as clear as possible. Plus it's incredibly helpful to have someone else read your manuscript out loud (my local groups take turns reading a manuscript from another member).
You can query publishers after polishing one picture book manuscript...but like others said, the more editors you submit to, the less likely an agent will want to sign you for it. Having at least three different, strong manuscripts is so important when submitting to agents because if they don't love the others as much as a first, it could be a no or they might ask for even more. I've been asked for as many as five after the original submission.
Definitely check out your local SCBWI. I'm lucky to live in an extremely active state with two large conferences, a one day Boot Camp, plus free two hour meetings most months in 5 areas of the state. Plus, you might have a critique group near you. It would be a great compliment to getting critiques here and/or an online group since critiquers who know your manuscript well can tell if you've taken a backwards stumble and fresh eyes are incredibly helpful.
Inexpensive SCBWI webinars are getting more and more popular. You can find a bunch of them on our board. I've taken two by Pat Zietlow Miller and she's amazing, and has another one coming up on Feb. 22.
Check out #PBChat on Twitter--there's a lot of great advice there. If you sign up on Justin's website, he'll notify you about upcoming chats:
http://justincolonbooks.com/about-justin/He has lots of wonderful classes, challenges, blogs, etc. listed on his resource page.
I'm a member of 12x12PB and love the community and motivation to try to write and/or revise 12 PB each year. They have amazing webinars included with the annual fee:
https://12x12challenge.com/membership/