So with a space of that size and with the frequency that dust (especially in our dry winter) ends up in the garage, it was not feasible to keep it perpetually clean. Adding the equivalent space of a small 2 bed apartment was also not particularly fair on my helper... even if it is 'just' mopping.
Of course my first port of call was to go with a Karcher robot vacuum/mop but pricing is fairly high, with the consumer model at around R8K and R13K for the pro model. I was also wondering about units with larger water reservoirs and other features since I'd never used one of these myself
One of my mates had bought a unit during lockdown and barely used it so kindly gave it to me to try out for a few days: Turns out it fit my use case perfectly, so I ended up getting it from him.
Setup and Automation:
Setup is simple. There are two tanks: One is a combo vacuum and water tank, the other is a plain vacuum tank. There is one spare brush and one spare mop microfibre but it is easy to find kits for this inexpensively.
The app works well but does require registration. Once in, it is easy to set up automation. The unit builds a map the first few times it is used. I have it set to vacuum every other day at 11pm. So far it has worked just fine.
Pathfinding & height
I had no issues with it finding its way underneath the M5, 4C or my mate's visiting Toyota 86
No issues navigating around objects but it did (on the higher vacuum level) pick up a microfibre I'd left on the floor (it sent me a message about being stuck... the cloth itself didn't stop it but clumped up and blocked its path at the 4C side skirt

). I found it had also picked up a piece of plastic that was from one of my LED skirtings I am trying out... this long and narrow packaging wrapped around the brush. My fault on that front but it didn't cause any issues.
It did try to escape once (probably my fault as I thought I set the boundary at the garage door).
There is a bump sensor, tilt sensors and a LiDAR unit (the goodie sticking out at the top which is not present on some models) that make sure it doesn't break anything (or break itself). the unit is quite robust and did eventually make its way around things I'd left out to be in its way on purpose.
The unit has not failed to find its way back to its little dock and will also charge itself if it starts to run low on battery and then resume whatever it was doing.
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Vacuuming
This unit has 2.1KPa of suction power. For tiles in the garage and the type of dirt coming in (leaves, flowers, pollen, dust, small stones, sand/dust) it is perfectly fine. There are units with 4Kpa but at the same time for my use case this will never be working on thick carpets. Medium setting has been fine. If I cared about noise even the low setting would probably be good enough for this. There is a small radius that it is obviously unable to get to but not a big deal. You can see this below in the corner. Not a biggie as R2D2 (my giant Karcher Pro unit) will have that sucked up in half a second when I'm vacuuming the cars
Mopping
You still have to use detergent and 'normal' mopping maybe once in 2 weeks. You can't use detergent in this unit at all - strictly water. This is not really a replacement for actual 'cleaning' but it does pick up a heck of a lot of dirt on the microfibre pad as part of this process. It is a little like a snail, injecting a bit of water onto the pad, leaving a bit of a trail and then going over it in a more 'zig zag' pattern than the vacuuming. Effective at keeping it looking perpetually 'fresh' but not a replacement for a deep clean IMHO. Mopping (of course) takes significantly longer than just vacuuming.
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What would I recommend?
For a few grand this was a no brainer to buy used. I have no doubt there are a number of these units sitting collecting dust from folks that thought it would be more useful to them in their flats/houses than it actually was. The batteries (should you need them for these 'sitting' units) are fairly easy to find as are the replacement spares and filters.
If I had to buy a unit new for garage use, this is probably the way I'd go with Lidar, similar brush system, better suction and battery. The water tank on mine was sufficient for the full garage on the high water use setting.
This is the difference between mine and the new equivalent replacement (approx 7-8K): I feel like the 'high frequency sonic vibration' thing is likely to not really be a huge deal. The same limitations apply in terms of detergent use and it is not going to replace (as mentioned above) a 'real' deep clean. Obviously for garage use mapping multiple floors is irrelevant to me but may be something one of you is after. Battery life I've mentioned above. I have no doubt you could max out the suction & water injection with the battery below for the 80/90 square space of a quad garage. The suction difference is (IMHO) irrelevant as you aren't even really using the max setting for a hard floor on the smaller unit. If you had carpets this would matter more.
Anyway enough reviewing. Hopefully this was helpful.