The title is kind of contradictory. If you want max stimulation, you don't have long sessions. If you want long sessions, you probably want less stimulation. Of course, if you could naturally last very long, it doesn't really matter how much stimulation you have. Since you mentioned you never tried onaholes before, I would recommend r20 by Toysheart, https://www.otonajp.com/r20
. You can control the stimulation by pushing air out of the onahole. If you want max stimulation, push as much air out as you can to create a vacuum and it will suck you dry. It has enough length to accommodate any size. And it is easy to clean and maintain. Not to mention, it has been an all-time favorite no.1 best seller in Japan.
I would prefer loose over tight. Many onaholes are just too tight for me, mostly due to the materials being not flexible enough. A loose onahole can still provide a lot of stimulation, if it could keep air out during your session, as in the case of r20.
If you want an opening on both sides of the onahole, it would be difficult to have a lot of stimulation. Most onaholes actually don't have openings on both ends.
If you don't pull out, you would contract heavily inside the onahole. Some might find the sensation too much. It depends on you.
Hope this is helpful. Please have a realistic expectation for onaholes.