Problem solved. Learned a lot about A/B partitioning in order to get out of this situation. I think I corrupted the OS by updating from Android 8 to Android 13 with only one install.
So my A slot was corrupted because of me. My B slot could not boot either, I don’t know why. I switched to the B slot. I downloaded an official Sony firmware using Xperifirm. Then, using Newflasher, I flashed all the B partitions with the official firmware from Sony. My phone could then boot on the B slot, leading to a stock Android 10. I checked the available updates in order to make sure I had an updated Android 10, as said in the installation guide. Then I installed the recovery exactly as said in the guide. I made sure to follow the instructions in the «Ensuring all firmware partitions are consistent», which was necessary because, remember, my A slot was corrupted because of me. This allowed me to get a valid A slot, just like the B slot. And then, from here, I just followed the rest of the guide as usual. My /e/OS is now working.