Leo looks so cute! Those ears

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Gorgeous border as well.
I know what you mean by the mousey smell similiar to that of some male dwarfs. Eddie has it about him. I reckognized it from Ozzy soon as he moved in.
Do you need insurance for small animal boarding?
I have considered it as a retirement project but worry about conflicting ideas of hamster care. If someone pays me to board a hamster who has a wheel that is too small for example, that would cause me a big problem. On one hand it wouldn't be my place to insist on a bigger wheel because i'd be providing a service i get paid for but on the other hand i'd feel obliged to consider the hamster's wellbeing.
I know from rehoming that some people are receptive to improving their hamster's lives but not all are.
I have business insurance, which is not a legal requirement but is a very good idea. You also may also need to update your house insurance if you are running a business from home and having business customers visit your home.
I have always found people to be grateful for tactfully-given advice about wheels and so on but of course it is a very different dynamic when you are looking after someone's pet vs rehoming and it requires sensitivity and some degree of humility. I try to phrase advice in a positive rather than negative way, e.g "he's a healthy-sized hamster and would probably be more comfortable on a bigger wheel" rather than "his wheel is too small".
You do ultimately have to respect people's right to make decisions for their own pets. There are times when I can tell from a hamster's cage setup and the types of things they have their cage that their owners have done research (e.g cork logs, multichamber houses, a bendy bridge over the entrance to the house - these are things that people generally only know about if they've been researching online) but there might be one incongruous thing, like maybe they have a Pets at Home XL cage but it's reasonably well set up and the hamster is content. Those cases are where the humility comes as you just have to accept that these are not uninformed or uncaring people and this is how they've chosen to care for their pet, and I don't believe I have the right to impose my own opinion. I try to reserve that for cases where there is real harm being done (or which is likely to be done) to the hamster, which the owner is likely unaware of.
Overall, I would say, don't overcomplicate the issue. Those of us who are active in the online hamster world, and who are familiar with the huge amount of neglect which some hamsters suffer, tend to view these issues in a moralistic way, and rightly or wrongly, can often tend to make assumptions about an owner based on the size of their cage, their wheel etc. That's why giving advice can feel so difficult or risky to us because we feel like when we say "his wheel is too small", we're afraid that we might actually be saying, or that the owner might hear "his wheel is too small because you're misinformed and you're a bad owner", because those the associations we have. But the average "offline" hamster owner who isn't familiar with the online hamster world, probably only hears "his wheel is too small" and doesn't attach to that advice all the moralistic stuff (unless it is framed in that way). I find when I try to detach things like wheel size and cage size from the moralistic associations, telling someone their wheel is too small is not much different to telling them their pet is a different sex to what they thought it was (which happens from time to time). It's literally just communicating a fact, and it may be met by surprise or even disappointment, but rarely anger if it's communicated sensitively, and usually also gratitude and a request for further advice.
Of course there are times when a hamster is so poorly cared for, even abused, that it's appropriate to make moralistic assumptions about the owner but I have never come across one of those cases personally, as those aren't the type of people who pay for pet boarding.