I finally got a sitemap set up and submitted it, and the atom feed, to Google. I also found out how to get a custom robots.txt file set up for a Nabble forum
(you do it with NAML). Here's our robots.txt file: https://www.growspice.com/robots.txt
Google made it easier to submit sitemaps. All you have to do is go to the search console and give it the URLs for your sitemaps. I was reading the help files, which didn't take that into account (so, that's why I was learning about robots.txt—to direct it to where my sitemap file was).
Anyway, now hopefully we'll get more traffic.
Hopefully, Google will automatically add all new posts with the atom-feed-thing—but, if they don't, I made a Python script I can use to gather the URLs and do it manually (which is what I did for the first sitemap).