While the bar for entry in the realm of self-publishing is pretty low (can you figure out a way to create a PDF? You're in.), there's a pretty steep learning curve when it comes to doing it well.

All the advice from the pros is right:
Focus on what you do best - if you're a good content creator, create; if you're great a layout, do layout; if you're an artist, do art... If you're good at a couple areas, go ahead... but know that the fewer eyes on a project, the more likely compromises and errors creep in.
Edit, edit, edit - There's a quote I read somewhere (for my other job, I think), that said a writing project is not done when there's nothing left to add, but when there's nothing left to take away.  I get it.  That's hard for me - I am wordy by nature, so having the wherewithal to see those darling words not as something precious to protect, but as possible fat to trim is powerful - and not usually in a writer's zone...
Playtest - yeah, this step is really important, and really, really easy to skip over...
Use the right tools - Layout software is powerful and does exactly what it says on the tin - allows you to professionally arrange elements on your pages with precision and with a minimum of fuss. So Word or Libreoffice or whatever - the "jack of all trades" word processing programs that can do layout but aren't really designed for it (and Word seems to HATE large, image heavy files, for those who want to know) - yeah, you can make them work, but that's not really their strength...

And lots of other advice.  Thing is, I ignored all of the above advice. That may tell you something about the products we're producing here at Rosethrone Publishing, I don't know.  I think it says more about me and my passive-aggressive attitudes toward people who try to tell me I'm wrong than anything else...

Lessons learned:
You CAN produce good quality products with the tools you have on hand, the tools you are super comfortable with. I have tried Scribus so many times over the past ten or so years - knowing that I'm "supposed" to use it, but I just hate the implementation.  I have used Word (or the like) since before Word existed - I was using Wordperfect and AmiPro decades ago - and pushing them as far as I could (I tried to replicate what was then considered professional RPG layout in AmiPro in the late 90s).

Thinking about the past several years of trying to put things together, I find it interesting that Layout is my favorite part of this process.  I like "having written" but the process can be arduous, and I always look at what I've written with a jaded eye. I will continue to peck away at Scribus, maybe I'll take an unformatted adventure (reduce something to just text) and try to do a layout from scratch.  I don't know.  I'm happy with my results, YMMV of course.

I've got some other things to talk about, of course... future reflections forthcoming...

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