How to enter surface-mount main panel from back w/ a 1/0 Al SER cable?
On a seemingly unrelated topic, I like to see houses have LOTS of spare breaker panel spaces.  Spaces are cheap, and in the future stuff always comes up, and nobody has ever said &quot;Oh no, I have plenty of breaker spaces available, whatever will I do?&quot;
Hold that thought.
Now, given the thickness of the cable, I do not see way to make that bend.  What I advise doing is installing a LARGE junction box on the inside of the garage wall. Now your SE-R cable can enter that box vertically, transition into individual wires, and go through a conduit nipple from this inside box to the outside box. At that point we're only concerned with radius of individual wires, and that's no problem.
Now let's price that.  Splices of large wires require very long, tall boxes.  We also need splice connectors for 1/0 wire (four of them)... Hold on a minute.  What's a LARGE box that comes with big lug connectors, that's cheap?
Why a subpanel, of course!
So if extra breaker spaces wouldn't be useless, the best answer may be to fit a large subpanel on the interior of the garage.   &quot;Large&quot; because you want full width, and enough height to handle the large wire bends. A 14&quot; wide psubpane gives you
And that's exactly what I'd do. Select a 200A-bussed, main-lug but convertible subpanel of 20+ actual spaces (not circuits).
I'd arrange the new panel back-to-back with the existing panel, using a &quot;nipple&quot; to connect them that is as large as the knockouts allow.
Downhill from there.
If the SE-R cable's internal conductors have their own markings (as XHHW conductors) then you can enter the new subpanel with a grommet/cable clamp, then pass right through the new sub and into the original panel - at that point you're just using the sub as a giant junction box.