As I read it, Excel isn't viable any longer at least due to file sharing issues because Excel does not do concurrent users very well (if at all). So no Master workbook or anything like that. I would also rule out maintaining the wb and linking to its sheets from Access because of that - unless you can have someone who maintains the wb and you link to the sheets. However, as stated, xl sheets usually make for bad related db tables. This means if you link to the sheets, you will most likely need db tables that are correctly designed and related and you port data from the linked sheets into these tables with mucho append and update queries. Then there is the code issue. Most of the operations that have been coded for Excel will need to be re-written for Access if they are still needed. Those that won't be needed might be replaced by things like queries against Access tables.
IMO the best route to take is to bite the bullet and build a proper db that supports the objectives while still using Exel in the meantime. There may be a bit of an issue wrt a lack of relational db and Access experience. There is a bit of a steep learning curve if one is unfamiliar. Much research should be done to get off on the right foot lest you continually encounter road blocks because of improper design. The #1 thing to learn first is db normalization. If you have vba experience in Excel it may help a bit with common things such as vba functions (e.g. Left, Right, Mid, etc.) but it won't help much with coding for the Access object model. Perhaps review the links in this thread and decide who is a candidate for taking this on or whether or not to contract this out.
Last 2 points I'll make for now regard the potential file size and db sharing. I imagine that you won't come close to the 2GB max for Access given that you are now doing this in Excel but that and the lack of web support for Access may be factors. Note, you must not attempt to share Access via the cloud or over wifi - there are other options if remote access is required.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.