In the last years I have used TTW Theming (uploading a zip) a lot. Therefore I have many small customer websites that are using this feature. Right now they a running on plone 5.2.x with python 3.
What would be the recommended setup for plone6. I guess in order to become future-proof I should go with Volto frontend, but that may be to much effort for small websites.
I would appreciate any recommendation even if the solution is to move small pages to another CMS.
I would say upgrading to plone 6 classic might be your best option. however I'm not sure uploading zips is still supported but converting to the file based theme is not that hard.
Volto makes sense if you have a larger budget or if you can reuse the same theme across many sites. Customisation of Volto involves overriding many react components which is a reasonable amount of work. On the other hand it is something someone who knows React already might be able to quicker than learning the plone classic way?
Later on there might be more support for Volto in a headless mode that would lessen the amount of work to create a theme if you know a frontend framework well already.
Or thats my 2c anyway.
Here is a (WIP) outline of the theming options that you can use in Plone 6:
For small customers, you might be able to use a TTW CSS stylesheet override.
Agree with @djay that converting zips to filebased themes in not that hard. Also consider that you will have to make some changes because of the newer bootstrap.
FYI. Uploading zip themes is still supported in Plone 6 Classic.
Two years ago I did a POC to create a Plone 6 Classic Boostrap theme without any knowledge of Python / Plone. The code for this POC is available here : GitHub - sverbois/plone6_theme_builder
@djay @mtrebron Thanks a lot for you quick answers an pointing me into a direction.
I think I can use yarn with multiple src configurations to build the necessary dist files for each website, based on a theming product. I am doing this with npm in other projects, I guess yarn can handle that as well. That may be a better solution as the TTW theming when it comes to updates/maintenance. I have to check ... 
Thanks anyway!
Thanks for the info. The question is for how long? Following the discussions on the Plone forum it seems to become obsolete, but I may be mistaken. I like the idea of uploading a theme and done but I also now that this comes with huge complexity. As theming a website is essential I prefer to use core mechanisms or if necessary plugins that are maintained intensely.
I would say, for as long as there are use cases for it and for as long as volto has no feature parity. There are quite some Classic UI users in the community, perhaps not as vocal as the Volto group - maybe because Classic UI is more mature? 
I was more refering to the TTW theming (@@theming-controlpanel) and uploading Zip-files. I assume the classical theme will survive at least Plone 6 
I think in that case you are correct: the TTW theming story seems to be reaching its EOL. I guess custom CSS is somewhat of a transitional band aid. Since filesystem theming is quite straightforward anyway I would personally choose that option.
you can still upload things in Zope ZMI portal_resources, that is what theming does in Plone 6 too. Theming is a bunch of files + the entries in RR control panel (theme switching/activation is still a little obscure to me). You can do it TTW in Classic UI but it is not easy nor clearly documented.