There is a totally different way to solve this issue, let me please explain my solution.
MS Access always loads the full content of tables into queries prior to running the query. Other way round, changed content of database entries will be written back into the table after you applied the changes. Your main problem is the write access into the database but not the folder on the NAS.
Divide your MS Access project into two parts, the frontend and the backend. The frontend contains all queries, reports, forms whatever. The backend contains the tables only! Connect the backend tables into the frontend part by linking the tables of the backend into the frontend. Here you can find the full story: https://support.office.com/en-us/article/Split-an-Access-database-3015ad18-a3a1-4e9c-a7f3-51b1d73498cc
At the end, use the built-in user rights management of MS Access to grant access to the database but keep the backend database in a special shared folder on the My Cloud. This folder only needs/grants access to one(1) user account. This NAS user account need not to be similar to an existing user account on your PCs, it’s only for establishing the connection between frontend and backend of your MS Access project. Please don’t mix this with the way ODBC “Open Database Connectivity” is working, ODBC is not applicable here as we don’t have Windows OS on the My Cloud.
The authentication account you setup on the NAS need to be used for the conenction to the .accdb file stored on the NAS. If you want to, you can even write a short VBA script to to the linking, so you can hide the login credentials to “normal users”. The frontend can be deployed on each client PC.
Splitting your MS Access project into frontend and backend offers you multi-user environment, a much better performance of your MS Access database and a huge decrease of network load - all in once at the same time.
Try it, it works perfect.