Let's focus on the lunar orbit idea as a relatively straightforward objective, since it is practically in our backyard and we have so much experience there.

I am not suggesting we perform 50 different launches into space. That would be hopelessly wasteful. But suppose we did one launch, sent one ship towards the moon, and then have it release 50 probes as it got close, each with a small amount of propellant sufficient get itself into orbit.

I am not suggesting a common design that is adequate to handle all the different conditions of the different missions you mention. I said specifically that we should focus some effort on developing cheap probes, accepting that they will be suboptimal in almost every case.

Let's consider one of the most basic things we like to do, which is to simply take pictures of things. Pictures help sell science to the public because most people are interested in how things look, and they are scientifically useful. When we aggregate multiple pictures of the same subject we often get even more useful scientific data. Downsides, the data transmission requirements are large and visual spectrum is just a small slice of the information we'd like to collect. Upsides, you can buy a COTS camera that takes 4k video or ~50mp stills for a few thousand $. Likewise you can buy a fast lens of high optical quality very cheaply, and record onto very cheap solid-state media. Let's accept that it will fail in some situations and that we don't expect it to keep working for ever, but we would like it to work for a while. so we need some power (onboard or renewable or some combination of the two), an antenna of some sort to transmit the data back and listen to requests from our end, some shielding to protect it against the slings and arrows of outrageous fortune, some sort of propulsion to get it into position and point it roughly where we want it to look, and a little control system to run it all.

Technologically this is no longer a tall order. We can stick a consumer video camera & phone in a lunchbox, attach it to a balloon, send it up to the stratosphere, and retrieve it afterwards for only hundreds of dollars, it's a middle-school project by now. I think that we could make a pretty decent camera probe that would take relatively high resolution pictures at a relatively low frame rate and last for at least a year for a marginal cost of $100,000, maybe quite a bit less. 50 of those would be $5 million, which is the sort of sum you can raise on Kickstarter. Now, the fixed costs of launch, building a deployment module and numerous other things would be a lot higher, let's say they started at $50 million. Well, that's quite a lot of money but you could still raise it pretty easily. Donald Trump plans to waste twice that amount on promoting himself as a public figure while pretending he wants to be President, a summer blockbuster movie has launch costs of about $200m including marketing. There are lots of people in Silicon Valley who could write checks for that whole amount if they really wanted to. I pick $50 million as a benchmark because India managed to get a probe going around Mars for ~$75 million, so I don't think it's a totally outrageous idea to think we could deploy a bunch of lunar microsatellites for 2/3 of that.

OK, let's say we even went overbudget by a factor 2 but we managed to do it. We have 50 probes in lunar orbit sending back, i dunno, 43 4k photographs of the lunar surface at the rate of 1 frame/minute (7 of them failed to deploy properly). None of them works properly for longer than 18 months. Within a few years they have all fallen out of orbit and are space junk on the lunar surface. Well, I think that we'd get a ton of useful knowledge from doing that.