Tohoku sights advise, train, shinkansen

Hi,
Looking at the north Tohoku map seeing some interesting places, how convenient are the Akita and Yamagata shinkansen compared to taking other train routes?

I mean to see places like Hachimantai, Dewa Sanzan, Shirakami Sanchi, etc...

When?
Off-season no operate seasonal bus after Shinkansen.
Do you have IDP? (for car rent)
https://www.toyotarentacar.net/english/guide/license/

Oops sorry, from somewhere around april to june.

Haguro-san is the only part of Dewa Sanzan 100% open until "late" April, when Yudono-san opens, but "late" April is pretty nebulous and I am pretty sure it wasn't open when I was there. Gas-san doesn't open until July. I went in late April '09 to just Haguro-san and then took the bus from Tsuruoka to Yamagata (and then continued on to Zao.) There are a few buses a day and it is not covered by any rail pass, but can be faster than trying to catch a Limited Express to Niigata and then taking a shinkansen, and is faster than the train from Tsuruoka to Shingo. I started in Akita and took the train down the coast, it was beautiful and I had a great view of Chokai-san but it was slow. There is only a single track in either direction, so a Limited Express won't get you there any earlier.

After getting caught in a blizzard near Towadako a few days earlier, I understood why places like Yamagata keep things closed until May or later. There was still a lot of snow on the mountains that the express bus traveled through, including Gas-san.