Indeed: Mint 21 is already being prepared. And there I thought I was behind. Mint 18 is no longer in support, AFAIK Mint 19 ran out of support last spring…
Following the article? But I have no idea if that still works on such an old version.
Um, what? Maybe all the time you just forgot to run fdroid update? fdroid deploy is for syncing the stuff up to your server. It won’t do a thing if there was no update run before (at least there won’t be a new index and thus no client would know to check for the screenshots etc).
Then I’m out of ideas, sorry. These steps sound correct – and provided your structure is correct, should lead to the intended result. In case we’re looking at the wrong tree, you could open the index-v1.json e.g. in a web browser and search (Ctrl-F / Strg-F) for one of the screenshot’s file names. If you find it there, all went well but your client looks at the wrong place (or you sync against something else).