I've never used the granulated product but I was contacted about it and tried to get some in the UK but it isn't being sold here yet. Like you suggest I wanted to get some in for any issues but I have to be honest I wasn't intending to use it routinely.
I think you need to back up slightly and think about what you're trying to achieve. Footwear is a risk of spreading contamination around your plant. Wet footwear spreads contamination more readily. Any disinfection process is readily exhausted by debris not removed in this cleaning process and there is always some with a boot washer. It's also worth thinking that you only wash the boots at the entrance or exit so it's not stopping spread from one area or another in any case.
For those reasons, many production facilities in the UK moved away from any dipping or bootwash processes or they've moved to only cleaning boots on the way out to limit excessive water, not all but I would say most. Many have also moved towards having someone who cleans the wellies thoroughly at night instead (or sometimes as well as). A boot washer can't use a decent clean as a person can, i.e. gross debris removal, detergent, rinse, disinfectant etc and a boot washer doesn't have decent contact time.
If you think about it, the risks of footwear spreading pathogens around the room can only be there if the pathogens are there in decent numbers and it is enhanced by the footwear being wet. So for that reason, I would personally stay away from the bootwashers whether they are small cheap ones or large expensive ones. I would certainly, always stay away from dipping boots in a tank.
As for the granulated product, it could work but the key thing is effective cleaning and clean footwear. It could be tempting to use it as a sticking plaster for issues when eradicating them with effective cleaning would be much better.
You're right to be concerned about pathogen risks with your product, Listeria particularly is one I'd be wary of and you need to control your facility to minimum high care standards IMO. You have the benefit that at least any Listeria monocytogenes can't grow in your product once it's frozen which is good but I'd literally draw your high care area on a piece of paper then draw on every single thing that goes in and out of your box. I'd include everything from people to ingredients, air, waste water etc, etc. Then for each step I'd work out how it is controlled to make sure pathogens aren't brought into your high care area or spread around. Additionally the drive should be that items which don't have to leave your high care area shouldn't, e.g. having captive footwear. Spending some time on this barrier risk assessment is more likely (IMO) to lead to improvements in Listeria control than a boot washer or disinfectant granules.