Is it in some way possible to create templates of pre-made parts and/or assemblies? (preferably the latter)?

As far as I can see templates are just settings, without actual objects in them.

But we use the same objects (in assemblies) every time to start from - which now means we need to "save as" every time... which is way too much work considering how easy it can be (especially on an assembly): because having to save as copy, rename the lot ( we don't use pre- or subfixes in our article/part names) etc etc. - still loosing connectivity and equation .txt files too often on the way.

What we basically need is what you can do in about any other piece of software: complete a file as far as needed to cut preparation work and save it as template - the basic reason why "templates" exist.

Whenever we try and save an assembly that actually would save time to use we get "A document which references other documents may not be saved as a template"...

An assembly by default, by definition, references other documents.... so what's the use if you can't?