18 days in August around Honshu

My girlfriend and I are currently planning a (rough) itinerary for our Japan trip in a few weeks and trying to figure out if the best way of getting around would be rail pass or a combination of seishun 18 / single tickets + we're also not adverse to a nightbus transfer a few times since we're on a bit of a budget.

Our rough plan with constraints is as follows:

-29/7 arrival in Haneda -first week travel around northern Honshu up to at least Akita (friend lives there and Kanto festival is happening). Tono area, Dewa Sanzan interest us from what we've read so far. -5-6/8 Tokyo since we're attending a wedding there (the reason for the trip initially) -second part travel towards Kyoto & Nara, visit Kansai region, up to Horishima/Miyajima if time allows, Onomichi region interests us as well. -16/8 departure Narita.

We will be catching up with friends in Tokyo/Yokohama as well so at the beginning or end or around the wedding will be spending some days in the Tokyo area (which I've visited before, unlike the rest of the country) but not clear yet when exactly.

So without having a detailed plan yet, I'm wondering what the most sensible option is transport-wise?
The 14 days JR pass makes a lot of sense since it would cover most of our trip (and we'll probably have tokyo days either at the end or beginning). But since we can also buy the seishun 18 bundles combining that with some single tickets and nightbusses for longer transfers could be cheaper, but would it be worth the longer time travel will probably take? We're not really in a rush, but on the other hand don't want to spend whole days in a train too much...
Flexibility of the JR pass is of course appealing, but that's something that the seishun 18 tickets can also offer (to a certain,slower, extent). In a way the choice of tickets/pass would influence the final itinerary of course.

Any opinions or different recommendations appreciated..
Thanks!