Shap - your engagement strategy is right on track - the only thing I would add to that is to make sure that you comment on what stands out as the "influencers" in your niche. Make sure you follow them and ask for follow backs...get your name/brand known to them.
Once you are ready to launch, if you have given them some love...they will be more likely to help you out even if it costs a little....much cheaper than anything mainstream.
Definitely set up a biz facebook....if for no other reason but to mirror the insta acct postings for the time being...once you have stock, the content should be posted at the same rate as you mentioned on insta...and then demo-target ad the heck out of facebook.
Learn Snap - make content for that as well