I know, I need help.
Honestly, you really seem to have most of the general principles down, it's just that your right on the edge of where a non expiring ticket might make sense. 10 day no expiration tickets make a lot of sense for people who typically take shorter trips of 2 or 3 park days at a time... and in that case you just keep using the ticket until it expires and then buy another one. As I showed above, though, once you get to 5 or 6 days it's kind of a wash.
However, the great thing about Disney tickets is that they're always upgradable within 14 days of their first use... and you can even upgrade them multiple times. The trick is that you can't ever downgrade a ticket, so start with whatever you know you're going to need and then just upgrade from there.
In your case, what I'd suggest you do is start with a 3 day ticket without hopping and set aside some extra money to upgrade your ticket with if you need it. If you decide that you really want to park hop one day, just go to a ticket window and add it. If you use all three days and want a 4th or a 5th then add those days on when you need them. If your kid is cranky and you only go to the parks for 3 days or you don't need the hopping option then you'll have never paid for them and you can save that money.
However, you will have a bit of thinking to do at the end of your trip. Since tickets are only upgradable within 14 days of their first use you will need to decide then if you want to add extra days or add the no expiration option.
If you wound up going for 5 or 6 days (or more if you add your arrival and departure days... remember that the 5th through 10th day on an expiring ticket are only $3 each, so even if you only go for an hour or two it might be worth $3) then I wouldn't bother to add the no expiration option.
If you only went for 2 or 3 days, but you really liked Disney and think you'll be back for longer trips in the future then I probably wouldn't bother to add the no expiration option then either. It won't save you much money to just buy a longer expiring ticket on your next trip and you'll have much more flexibility because you won't be locked into the non expiration ticket.
However, if you only went for 2 or 3 days but you really liked Disney and think you'll be back for more 2 or 3 day trips then you might want to consider upgrading the ticket to 10 days and adding the no expiration option. As you said, it's a big outlay at first, but if you will be taking more shorter trips in the future then it will save you money in the long run.
Finally, if you do decide to upgrade to a non expiring ticket, you might want to consider whether you want to add the park hopping or water parks options, even if you didn't need them for this trip. As I said above, tickets can only be upgraded within 14 days of their first use, so if you have a non expiring ticket and want those options for future trips then you would need to add them during the first trip.
HTH,
Brett