Migrating Content between sites

Hi all,

I have been developing a D9 site building content types, theming, configuring, etc, and we want to deploy different versions of the site (running on mostly the same framework) to different companies we serve (we would regularly add additional files to these sites, but wouldnt host them on our own server). Additionally I need to develop an app intermediary to bulk upload files/images to any given site in that umbrella. 

One thing I am noticing is that, unless two sites are basically identical, that just trying to push content or copy database tables almost always results in bad things happening and errors all around, understandably with config changes affecting things much more than expected, even if it isn't directly content related. 

All of the sites should be using mostly the same framework (a couple core content types) but might sometimes have slight modifications to the content types, a module here or there, or similar. The content types we would want to push out would have certain things consistent, such as:

1) Only the title/name are required fields

2) They will all have at minimum, a certain set of fields (these would be the only ones we would push, e.g. item name, item category, etc)

What's a good strategy for getting semi-identical content migrated across sites like this?

I have looked all over and it looks like most modules for this are no longer maintained/updated for D9. Features might have some of this, but moving a feature to a site configured different could override certain configs that I would not want to happen, and it also installs a module to the site which I don't think is the best idea. I would also need to bulk upload content via the app, so I cant just push a features module update everytime I want to upload a few documents to the site. 

I do plan to use Feeds for some of our content types, but this likely won't work for Media types that have files on them (since its a CSV upload) and we would want to upload media alongside various Fields (category, etc) that are not part of the EXIF data. 

I want to let each site have a degree of variation, but also a core content framework we can communicate with reliably.

Any suggestions?