Bike for Shimanami Kaido without reservation

Hi! I work in Kyoto until September and want to do the Shimanami Kaido some time in August.
Since I do research at a university and have a very unpredictable schedule, I want to take a day off when I've finished a run of experiments. I definitely want to make it a weekday so there are fewer people, but I have no idea when I'll have a day or two to spare. And I plan on starting from Onomichi.
Judging by the japan-guide page on the Shimanami Kaido it seems that there are bike rental places where one needs to reserve in advance. But usually there are places around that have no-reservation bikes available. But this is a very popular spot, so I was wondering whether anyone has experience with renting without reservations.
I plan on taking a 5.20am train from Kyoto, so I'd be in Onomichi around 8am to have the best chances of getting a bike.
Is this possible?

Thanks for your help!

(PS: I know it's hot and humid, but I've gone on bike tours everywhere between Ishigaki and Rishiri, mostly in the summer months and know what I'm in for ;))

I rented a bicycle at Onomichi Port a few years ago.
There were so many bicycles.
It is unlikely that all bicycles are already rented out at 8:00AM on a weekday.

The website says they have 900 in total.
http://shimanami-cycle.or.jp/rental/english-reservation

Thank you!
that sounds good.
Even with a mamachari, I think I'll manage somehow to get across.

I did it in late May without reservation. I was just told to show up by 7am at Onomichi bike rental and it worked fine. I also wrote about it here if it would help: https://bizarrejourneys.com/shimanami-kaido/

Thanks! That is a very informative post!

PS. Interesting part (and I agree) on "ugly" architecture. It's my 5th time visiting Japan for extended periods and I might come work here permanently, but I've never come for the glossy advertised version nor did I expect to see just beauty.

Definetly. I highly recommentd Lost Japan by Alex Kerr or Dogs and Demons by the same author. Both are very informative books about the construction policies in Japan.