Computers are good enough source of randomness for that purpose if used right.
I'm not saying use an online generated key as is ;)
You can generate 10 supposed keys off a webpage (or webpages to mix it up more). And then throw dice to determine what segments you get from each key. You get the building blocks in the right format so you don't have to convert and then you randomize with the dice. Sounds secure enough to me and yes, the page owner can't really know what you're doing in that case which is what I was referring to.
But I also see no harm in being a bit overly paranoid about something like this, as you said, if it's your savings, you're always better safe than sorry. I just see your method as a tiny bit of overkill.
Still, it's really nice for you to share your worksheets with the community and I'm sure people are going to make use of those.