I managed to upgrade a Plone 4 site with various addons and customizations to Plone 5.
All upgrade steps went through in the end ... but no css and js from Plone 5/Barceloneta is loaded.
What profile should I reapply to somehow redo the basic Plone 5 setup?
Does your custom package define a theme or not?
yes it does
You should put:
<xi:include href="++theme++barceloneta/rules.xml" /> in your rules.xml
and @import "++theme++barceloneta/less/barceloneta.plone.less"; in your less.
I don't even have the toolbar...
You probably know this already, but if this is local development... visiting the site at 127.0.0.1 will always return an unthemed site.
You probably know this, too:
If upgrading a Plone 4 theme you need to add
*section id="portal-toolbar">
and
*replace css:theme="#portal-toolbar" css:content-children="#edit-bar" css:if-not-content=".ajax_load" css:if-content=".userrole-authenticated"/>
To load the toolbar
Is it possible plone.app.theming never got installed? Try going to /manage -> portal_setup -> import tab and import plone.app.theming profile
There seems to be more stuff missing... portal_resource for example and some more old interfaces... who remembers formmailer? 
I think the instance is just to worn out, originally coming from Plone 2.5, so i decided to go the jsonmigrator way.
Just wondering which profile in portal_setup is used for the initial Plone5 setup, because I didn't find that one.
That's Products.CMFPlone:default and Products.CMFPlone:dependencies, but you do not need to mention them in your package, because they are installed when you create you Plone site at start (sio before you install your own package anyway).
Thanx, that was what I was looking for!
Products.CMFPlone:dependencies with "Apply all profiles. Already applied profiles are reapplied."
Didn't help either, will go the jsonmigratorway.
p.s. Maybe good to mention that the Products.CMFPlone:default with the above option will wipe all content as well.
@agitator,
Just finished doing a jsonmigrator + custom importer migration.
My script reads the outputted json files and populates my Plone site with dexterity items.
I used zexps to move Ploneformgen forms and wrote a custom yaml configuration for my portlets.
You can private message me if you want more details.