Ubuntu 22.04 LTS (Jammy Jellyfish) released

It sounds like you've got into a sucky place, so please, please take this just as constructive criticism you can take to management as an outsider's view, and not as an attack - I'd like this to be ignored at worst, and helpful at best.

The first thing I see going wrong is that the list of questions I was sent is huge, and the recruiter was unable to help trim down to the set that are relevant for me, instead recommending that I answer the full set in a big essay. That is unfair - I could write an essay answering all the questions, but some of them require you to have several years of experience, while others are asking about stuff that happened decades ago and is no longer relevant to who I am today. A fair process would give guidance on which questions you want answered given my history; someone you want who's switching careers at 40 would not be able to give good answers to all the questions I could answer well, while someone who's just finished their Comp Sci degree at 21 would be able to give better answers than me to some questions (because they're talking about the last 5 years, where I'm talking about something that's over 20 years ago).

This is something other companies fix via web forms; you get given a pack of questions to choose from, and the system only allows you to pick a limited set to answer, doing its grouping so that you can't focus on one area, but instead answer questions that give a clue about all the areas you're interested in - maybe one group for "can you code", another for "are you able to form part of a team".

The second is that the entire recruiting process is two-way; there are 7 billion people (give or take) on the planet. Of those, there's a chunk who'd never apply, a chunk that you'd reject at some point in the process, and a chunk that you'd hire if they went through the process at a time when you have a vacancy. The goal of a good recruitment process is to get everyone in the "would reject anyway" pile to not apply, while keeping everyone in the "would hire" chunk interested in following the process; however, because this process is weighted such that you ask a lot of people before telling them what the vacancy is (beyond "software developer at Canonical"), you're likely to put off a decent fraction of the "would hire" chunk, while not putting off that many of the "would reject anyway" pile.

Related to this is that Canonical is competing for talent with other companies - if your process is too heavyweight, some of the "would hire" pile will find jobs they're happy with elsewhere (e.g. at Collabora, or Amazon Web Services) and not be willing to put effort into your process. On the other hand, the "will reject" pile at Canonical almost certainly overlaps with the "will reject" pile at Collabora, AWS etc, and thus the people you don't want will still be willing to put time into the process because they don't get jobs elsewhere.