With the exception of Dark Souls, much of the delays can be explained. DQ was chucked up to the version of the engine used not being compatible during the game's original development. Now that the engine is compatible they have move the game over from the older version of the engine to newer release. Similarly, it is likely that development on Valkyria 4 started later, considering Switch was still new when the game was in development.
While your statement about porting PS4-based games does hold up for more graphically intensive titles designed around PS4, the Dark Souls port is based on the last-gen versions of the game. In fact, DS for Switch is being handheld by completely different studio than the PS4/Xbox One versions. There have been several ports of PS3/Xbox360 era games already that run and look better on Switch than on those systems. Based on the PAX demo, where areas like blighttown, which the last gen consoles really struggled with, were running fine on Switch. This indicates that this is not a case of the system struggling to run the game, but it could have to do with other things (maybe the online component of the game, or perhaps it was a business decision based on Nintendo filling up May with first-party releases which Bamco did not want to compete with).
Finally, I don't think your argument regarding sales of third-party titles hold up. We have had games like Attack on Titan 2 that have done fairly well on Switch, and even some late ports like Xenoverse 2. The problem right now is that we have very few major third-party titles that have had simultaneous releases with Switch. We know very little about how such titles would perform on the system.
I think third party games can do well on Switch. As you said, there is noted successes on the Switch, despite a majority of the third party games being ports. Heck, Ultra Street Fighter II, which was a $40 port of a classic game and its HD remix version, sold above Capcom's expectations. Then you got Doom, Skyrim, Mario + Rabbids, and others that have found success on the Switch.
There is opportunity for developers to find success on the Switch, especially since its selling well and has different userbase to tap into compared to PS4/Xbone.