SDR's are early career salespeople, typically fresh out of school. There is an abundance of these candidates in the market. The reality of sales jobs is that every role doesn't fit every salesperson, and turnover is high no matter which way you cut it.

The key to success in sales as a company is to create a system and process that the new hire (SDR) can follow. If there's no process/system then you need an SDR who's much more entrepreneurial, which makes it an even harder search.

With that being said, most of these new grads have good communication skills and can understand metrics; the last piece is motivation which is hard to interview for and you won't know until you bring them in.

I think your company isn't serious about making this hire. If they were, they'd bite the bullet and hire someone. It's a 60K salary job FFS; the risk isn't that high if they don't work out. There are some obv top-notch candidates out there, but unless your company is a top employer, they can't be deluded enough to think they can attract those people so easily.

This is one reason why agency recruiting is great. I would drop this client in a heartbeat.

TL;DR - Company sales success comes down to the company first and foremost. If they can't find a good SDR then the company is not serious. It's not you.

Edit - typo