Can I Start My Cover Design Before Finishing My Content / Book?

Not recommended. The reasons “why” require a shift in perspective versus common misconception among selfpublishers:

A cover is not an illustration for your (e)book. It is first and foremost “Product Packaging.”

About Product Packaging

Yes, your publication is a “product,” even if made available for free. Assuming this perspective immediately reveals the proper order of work to be accomplished in phases. You cannot package a product until after the product — your content — is finished and finalized.

Print Edition

Aside from a cover's look, the publication's trim size, paper type/weight, and physical (not numbered) page count must be finalized and known. These three attributes are required to calculate the spine [panel] width in cover design. There are no exceptions to this.

eBook Edition

It might seem that the above considerations do not apply to an ebook cover; think again. Whether or not you produce a print edition, you might change your mind later. Don't be too quick to claim “never” in this.

Content developed for the back cover will be used for metadata in an ebook. Yes, it can be developed before or after ebook content is complete, but it is better to do so after. Again, this dimension of “packaging” data should wait until content is finalized. Even for an ebook, leave cover development until last.

Finally…

Properly ordering work to be completed by consultants will also produce efficiency and in turn financial savings. Professional consultants do not work on set fees… not for everything. If you do not plan properly, you will increase hours (days) to publication completion and thereby increase your financial cost.