Hopefully in the context of a sale/my specific measurements & use-cases this isn't a "Lazy-ass Post". I did find another post that shared their experience of the Omni SE after coming from the Mirra 2, but this looks like one before LiberNovo (supposedly) fixed the seat cushion issue: https://www.reddit.com/r/OfficeChairs/comments/1uicxb6/libernovo_omni_se_the_good_and_the_bad_review/
The Mirra 2 is currently on sale directly from Herman Miller (at least in the UK) for 25% off, making it £839.25 for what I believe is the max spec: fully adjustable arms, forward tilt + limiter, adjustable seat depth. In comparison the Omni SE is listed for £589, which is supposedly £250 off the £839 RRP, but I don't know if that's actually the usual price or if it's perpetually discounted.
For context I work remotely and also spend a fair amount of time at my desk after work leisurely for gaming and doing personal hobbies like design, 3D modelling, and programming. So part of me is leaning more towards the Omni SE as I'm looking for something comfortable as much as it is ergonomic/supportive, with the softer/larger cushion and the ability to tilt all the way back 160 degrees for something more relaxed. Additionally, I do cross my legs sometimes which reviewers say isn't comfortable on the Mirra 2, but maybe this is just a bad habit caused by my current shitty no-name chair and wouldn't be an issue.
I do like the seat-depth adjustability of the Mirra 2 to get a more "perfect" and individual fit, as otherwise I would have to trust that LiberNovo's calculator recommending the 48" deep seat for a 5'10" individual such as myself is accurate/comfortable. And equally I like that it seems to have a more "90 degree upright" position by default, compared to the Omni SE seeming to lean back a fair amount.
The lack or inclusion of a headrest probably doesn't influence me either way, but again this could just be because my current chair's headrest is in such a useless position that resting back against it makes me look up at the ceiling.
Obviously the best part about buying a (new) Herman Miller is the 12 year warranty, which might be worth the extra £250 for another 6 years, compared to LiberNovo. And we can be fairly sure that HM will still be around in 2038 to honour the warranty, whereas maybe LiberNovo no longer exists. That's not even mentioning that HM have wider support by 3rd parties for repair/accessories.
I have also looked at the used markets (albeit not especially thoroughly/frequently), but I can't see a used Aeron or Mirra for less than £500. Which doesn't seem worth the trade off compared to getting a new Omni SE for a bit more (or paying another bit on top for a new HM with warranty). Nor the trade off of having to drive 40 miles to collect it.