Thanks for posting this thread, Gadget. I have read it, but remain unsure as to what to do with my DNA matches to progress further.
I will continue to work onwards - at minimum it is ensuring that my tree is as wide as possible to pick up the remoter cousins and their connections to our shared ancestors.
Based on VERY recent experience, I would recommend uploading to other databases such as Gedmatch, FamilyTreeDNA and MyHeritage.
About a month ago a new match appeared to a couple of kits I manage at Gedmatch. She was about the 30th closest match. I clicked on her and I knew several of her top 10 matches, and I could see that she had 11 matches from one branch of my family. From the mutual matches I could see that the connection had to be via a couple who married in Fife in 1815. She was matching descendants of two of their children, so she had to be related to at least one of this couple. Also, with the chromosome mapping you get at Gedmatch, I could see that she wasn't matching all of them in the same spot. She matched one fellow on a couple of chromosomes, a few on just one of those chromosomes in the same spot, and a few others on another chromosome in the same spot, which suggested a more recent connection whereas if she'd matched us all only on one segment it could be further back.
I contacted her and she has a couple of branches of Fife ancestors, but she didn't recognise our surnames. I built the tree back a bit for one branch that seemed the more obvious one to me - nothing jumped out at me. Then I left it for a couple of weeks.
Over at Ancestry she matched one of my three relatives who have given me access to their kits, and they share less than 20cM, so had just the one mutual match at Ancestry. Gedmatch showed more mutual matches who'd tested at various companies.
I had a look at her tree again a few days ago. She had a gt-grandmother born in Fife but the parents were unknown. I decided to get the marriage cert, so 6 credits later I discovered that this gt-grandmother's mother had the same surname as my gtgtgtgt-grandfather who married in 1815, and the matches gtgt-grandmother was born in the same village as where the 1815 marriage took place.
Unfortunately the parish records are a little scanty in places (or else, for some reason our ancestors just weren't in them - at times they belonged to the Associate Church) and the 1841 census is lost but we have a hypothesis about how they're connected which is looking sound.
If she'd not uploaded to Gedmatch I doubt we'd have even noticed.
