I've consistently advocated for a win-win mentality and respect between landlord and tenant but that is different from having one party do better at the expense of the other. I'd agree with the overall tenor of the posts that raising the rent seems not only fair, but overdue. As you proceed, I'd ask yourself a few things:
What do I want from this business relationship?
It seems you prize reliability, low maintenance and long term occupancy. In addition, these are some of your favorite "clients" so you wish to make life as easy on them as consistent with profitability and your other goals. You have an informal relationship about repairs you are happy with, though many would tell you thats a dangerous road to go down trading work for rent. So far so good.
The tactical question maybe is not "how close can I get to market rate" but "what is market rate for the kind of tenant (these) that I want?"
On the "what is the real cost to you side" the low maintenance costs hint to me of deferred capital expenses you will have to cover. Also, if tenants have been there a long time, no matter how good, when you do changeover you will likely need updates, etc. to get the so called market rate.
So I'd choose the price that allows you to continue the type of tenancy you have, then phase it in. 5% a year isn't a shock to a budget in the way 10% is.
I would not ask them, I would tell them. You are likely to cause much more hurt and bad feelings (and therefore damage to the business relationship) by asking permission. You said 80$ is reasonable and even on a landlord forum some people went "yikes, too much", so the tenant is likely not to have the same take on things as you do.
In a way, asking them permission is actually being less than respectful of their position as tenants. "So the rent is this but since you can't afford it I won't do it because I'm a great person". It feels good to you but in a way you are making them crawl before you to keep their home. It might not feel so good to them.
Figure out what you need then inform them--nicely--you are going to charge it. Do tell them your goals of keeping it low/below market because of how much you value their tenancy and their respect for the property. If they have an objection you find valid deal with it then.
I do respectfully submit this is about your comfort zone, not ethics. You want more money. You feel your price is too low even for the low maintenance tenant model you have. Own that and weigh it against the good things you have at this price point.
I have found that business which tend to be the most ethical and best to deal with are the ones which say "this is our price, its fair, and it is what it is."
Good luck and would love to hear how it goes. You have clearly done a great job of getting great tenants.