@C_Z i think my question with the Solar thermal is what exactly is the longer term use case? It seems like a stopgap measure to me that would be useful if we needed to send an interplanetary probe next year or the year after, rather than an impactor.
Why shouldn't we just wait a hot second and look to ion drives which have much higher ISP?
I'd also like to see all-sky in one of these but I'm not sure how to fit it in if anyone has any ideas.
Solar thermal is useful for sending probes, impactors, and basically whatever we want on a high-energy trajectory after already placing it in LEO. Shot-term, that's our lunar and interplanetary probes, which will continue being a niche that this can fill basically forever; they're still occasionally brought up for that role, but nowadays we send probes so rarely that you might as well just use what you already have. It's also useful for putting stuff into GEO; you get a fair bit more up-mass than with a chemical upper stage, and unlike an ion drive, you can get the satellite there in a few weeks instead of a few months.
Ion drives have an order of magnitude lower TWR and require extremely complex burns, and the specific impulse isn't
that much better for 1950s capabilities. KSP lies by making hall-effect thrusters have enough thrust to not make you go insane, but actual burns using ion drives are slow spirals outwards. The justification for solar thermal is the same as the justification for continued use of chemical vacuum stages despite us having used ion thrusters IRL. To put it into more direct KSP terms, solar thermal is like nuclear thermal but smaller and without the scary trefoils (as long as you're not too far away from the sun).
That said, it shouldn't be considered "ion drive or solar thermal", but IMO the two complement each other very well. Solar thermal's biggest development hurdle is the collector. If you already
have a big mylar mirror on the spacecraft that can concentrate sunlight onto a point, why not use it first to directly propel you with a solar thermal rocket, and then direct it at a photovoltaic panel and do midcourse adjustments or station keeping with ions. This matches with irl use cases; probes like Dawn used chemical kick stages to get them going to GTO or escape trajectories and then the ion thrusters took over. Solar thermal is competing with a PAM-D.
TL;DR, you have three different propulsion types, each with their optimal niches. Chemical is great for the high-thrust burns that you need for entering LEO or doing landings. Solar thermal is great whe you're already starting in LEO and need to speed up to go somewhere. Ion is great for after that kick, when you want to make very low thrust maneuvers.
