Not much to add but I wanted to thank you for sharing your story and describing your frustrations.
A good outcome would be that you get to a successful Discourse community. A great outcome would be that Discourse is tweaked to make this journey easier.
For myself, I made as few changes as I could and added only one theme component, and not immediately. I had the advantage of a relatively technical target audience, but I know that at least one smart person felt baffled by the interface.
I suppose my approach was this: don’t aim to recreate what you had before, aim to make something which works for your community. I wanted relatively low friction and I wanted people to be able to find what they wanted and contribute.
Part of my minimalist approach is to use rather few categories, and initially not to worry about tags. One friction point for the new poster is which category to choose, so the first one is always a catch-all and I’m explicit that the mods can re-categorise if needed.
I experimented with the default view, the front page can either be the categories or the latest. I only tried those two, and switched more than once.
The only thing I added was Topic List Previews
Theme Component which allows you to customise the layout for many of the Topic Lists across Discourse, fundamentally adding thumbnail previews and excerpts
What this does is adds text snippets and image thumbnails to each topic in a list, making the list of topics more attractive and more self-explanatory.