Excel as a front end? How would that solve the concurrent users issue? To elaborate on what Orange said, Excel data is wide (data is related across columns) whereas db data is tall (data is in rows and relationships are created amongst tables). I can only see Excel working with tables that are designed like spreadsheets, and that would be worse than what you have now. IMO your only real options are to stick with what you have until the single user constraint and sluggish performance becomes too much to bear, or bite the bullet and redesign now. I'd create a new application with Access as the front end and if desired, use Automation to push data into Excel for charting purposes. Unless you only need bare bones pie or bar charts, Access charting is woefully inadequate.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.