I would argue that waiting for FTL is a very good strategy.
Its not there because you can no longer ignore them, but it means they are ready to leave their star system and are ready to joing the galactic community. Its also a kind of a test, as developing an FTL requires certain understanding of both science and technology, and as Harkonnen explained very well, this insures that those people have a sufficient level of intelligence to be an equal participant in the galactic community.
Also, its not really ignoring them. Its shielding them from the more advanced technology and problems that they are not ready to face. Also, they have rights. They have rights to live and to evolve at their on pace. Any such world in Federation teritory is protected from any interference by a FTL capable species. Otherwise, they would be enslaved like in Stargate universe.
Very nice write up. And i agree with most of what you wrote.
I would like to add this. There are cases where interfering would help, and it would produce a best outcome. But, the problem is that it could go extremely badly as well. Do you risk and try to help, or just do nothing?
Its also a problem in todays courts. Are the people who dont intervene when they witness a crime criminals or are they in the right? This may be easier to answer and argue. But when you deal with a completely different species and culture, who is to say that your morals are correct? Who is to say that they would even want your help?
You could literaly play god for these people as we see in few episode of TNG and even Orville. I think PD is there to ease the decision for captains. To relieve them of the moral choice. PD is a precaution, its not always the correct decision, but objectively the best general rule there could be.
Nobody wants the moral problem equvivalent to the trolley problem. Prime Directive offers a way out for captains, to transfer the resposibility. It may not be perfect, but its best considering.