In light of this new information, I would like to reword my statement above.
As this is an experiment for which you have asked feedbacks for, here is my reaction upon seeing it above, and my more detailed feeling of it now.
I will start with the overall layout and my feeling of it, and then touch upon it's accessibility and usability, as seen by me and my experience as a user.
The reason I am not on Twitter and have barely touched Mastodon, is because of this layout. This several columns layout with the "main" one in the middle and squeezed between two other "side" columns feel oppressing, claustrophobic and cluttered.
Due to this layout I have been leaving the Tumblr tab more often than I used to, and it makes me want to come back less and less.
As well, I do not like the create button at the bottom of the column on the left as it feels kind of thrown in there and keeps catching my eye despite me, which is very frustrating and annoying and makes me leave the site faster.
This also goes for the "tabs" feature at the very top, which I wasn't using in the first place but feels icky to me.
This layout feels very counter-intuitive, as everything has been switched. Long term users and disabled users relying on muscle memory will have a hard time using the website now.
Now, I imagine you think "well, they'll learn", but this is another bad change amongst other, one that literally makes the website impossible to use. It would be the last straw.
I have caught myself several time moving my mouse to the places were things used to be, and felt frustrated when they weren't there, which prompted me to close the website and do something else.
As stated before, the "create" button at the bottom is very jarring and makes it uncomfortable to use as I have trouble focusing on the actual posts I'd like to see.
This is one of the reason I liked Tumblr layout more than other website, for it's clean slate look with just the post in the middle, allowing me to easily focus on these and scroll and actually stay on the website, which I'm assuming is what you want.
If you must absolutely go with this design, then I suggest dimming and moving the create button on the right, switching back the search bar and tool bar to left and right respectfully, and removing all the text next to the tool bar, leaving only the icons, maybe with the text appearing when hovering, so it would appear less cluttered and stop battling for my attention with everything else.
TDLR; This layout is overwhelming, cluttered, frustrating, counter-intuitive, and makes me leave the website more often.
