My response is going to be significantly different than anything else you see here.

Which is why before I share it let me tell you about my experience so you know where I am coming from. Since 2010 I have worked with over 50 B2B companies helping them reach, build relationships, and ultimately close their customers. Over 20,000 total customers reached through my efforts. Value of each customer starts at a few hundred thousand and higher as my clients were not selling B2B SAAS subscriptions.

Here are problems with the solutions you discussed:
-referral systems are inconsistent in providing quality prospects (customers don't know what profile you are looking for) as scale
-advertising and content marketing both take some time to crack (as you said)
-baller BD guys may or may not work. Baller BD guys have relevant networks of people who trust them but their past results may be a result of being part of upwardly mobile hot companies and products rather than their skills at positioning new products as a fit for your customer's needs (I say from experience of having hired someone like this).

So what is a poor newish B2B company to do? Especially since you already ruled out SDRs.

I would suggest the issue is not SDRs and lead generation firms in general. The issue is how you explained what you wanted and trained them to getting the best results for you.

From your message it seems like you defined the problem of outbound lead generation as two jobs--prospecting and scheduling.

Scheduling is only difficult if you are interrupting the prospect by forcing them to talk to you about your product rather than their needs and goals. As a rule the account exec should always handle all parts of initial engagement, curiosity generation, and getting someone on a needs analysis call.

Now let's talk about prospecting. Here are the biggest mistakes I have seen B2B companies make in training lead generation providers to give them the prospects they need:

1. Not clearly defining the prospect--most companies simply share what they do and who their existing clients are. To get quality prospects you must define this granularly to ensure someone with little to no background in your market, your products, and your customers can hit the ground running. An example from my business--if I say SAAS B2B companies to an SDR I will get a mixed up low quality prospect list. Saying this instead would get me much higher quality prospects: 1) B2B companies selling post launch offerings for app developers. 2) Each of their customers represents thousands of dollars in revenue (not a monthly subscription product) 3) B2B have established intent to buy products like mine.
2. Segmenting ideal prospect too broad or too narrow--this is a symptom of not knowing their own ideal prospect. The result is an inability to bring prospects you do get to close. After you get the prospect you should be nurturing the relationship with them via your marketing and sales function (not trying to do a fire sale right away). All lead nurturing by sales and marketing must be aligned by prospect segment. Eg. companies that say they want to work with everyone (too broad) or top 10 players (too narrow) often don't know what to do uniquely for each segment to get them to eventually close. You can blame the SDR then but really your internal functions weren't set up to treat each prospect type uniquely.
3. Cold calling--I abhor cold calling as do your customers--people view it as spam and an interruption. There is no reason why in today's world anyone should cold call. Proponents will say there are some situations where it works. Sure if you were the only one of a kind in your market and your buyer had limited information that would be the case. This is the reason why it worked in the 1980s. You would be hard pressed to find low competition, low buyer information markets now. In every market there are a small number of connectors. Building long term valuable relationships with these people the right way (by giving them a lot of value first) is the key to getting them to opening the doors for you to your market.

Do any of these sound like mistakes you have made?

I've gone through a lot at a high level.

Each of these is itself a process and system that must be thought out carefully for your market BEFORE you can use a SDR/VA or anyone else to help you with prospecting. If you are interested in getting help setting these up, let's get on a consulting call.