My thinking lately has tended towards “simpler is better”. A lot of time and effort can be spent on universal technically-correct solutions, but if the designers continue to keep their own project directories on their own local drives, the effort will be squandered. Of course, this all depends on the authority structure of the organization and the design team, the maturity of the project team and whether the leaders believe in process, how much effort can be justified for non-revenue-supporting “doc-control” work, where decision making authority is rooted (Mechanical engineering? Electronics engineering? Manufacturing and the ECO office? Product management? Accountants?), etc.
If “nothing works together”, the most pragmatic solution might be to not try. 
I agree ERPNext is not the place to store the artifacts for any significantly complex design. For purchased items, dragging a datasheet or catalog page for a COTS part onto the Item master may be more than adequate. Handing relatively simple custom parts in the same way may be practical. However, at some level of complexity clarity will be lost and day-to-day use will become problematic.
For me, it’s usually been enough to capture the design structure and supply chain details in the ERP, and reference the design artifacts stored in a separate PLM. The technology behind the PLM (e.g. Oracle PLM, Nextcloud, or a plain old file system) isn’t as important as preserving truth and a well-defined process for change management. Cross-referencing between the two can be as simple as using the same part number format in both systems.
It would certainly be convenient though if there was a link (button) in an Item Master in ERPNext to access the design artifacts associated with the Item in the PLM. Presumably any web-based PLM could be used if the link was a URL (ERPNext may even already provide such capability). It should however be difficult to access newer or older revision level artifacts by accident (but still possible if needed).
I like that Nextcloud integrates into the local file system, since in my experience that’s typically how designers work. The Nextcloud agent can be used to map the local project directory into a larger project space or master PLM space on the Nextcloud server. Like you, I expect Nextcloud (or a Plugin) will provide an approval workflow. The Nextcloud agent doesn’t have to be used though, and designers could instead drag design artifacts into an appropriate folder using the Nextcloud web client.
Bi-directional synchronization will come with edge cases, and could easily be a black hole of time for your bright young lad. The problem can be simplified enormously if a one-way workflow can be accepted (e.g. creating a part in ERPNext could cause a part number folder to be created in the PLM ready for uploading artifacts). Areas of friction can be addressed when they come to light.
Creating a standardized process for design re-use using different CAE tools will also come with edge cases. It may be more pragmatic to always save a couple standard format artifacts that others will likely find convenient (e.g. a JPEG/PNG/PDF rendering, perhaps a STEP export for custom mechanical parts, a zip archive with Gerbers for a PCB), and deal with anything more complex such as migrating the design to a different CAE tool, within a project when the need arises (rather than having the PLM carry the burden of design re-use if re-use is really more exception than practice).
Of course, IMHO and YMMV…
Cheers,
Dale