Uplift[0] also sells treadmills and exercise bikes specifically for standing desks. I purchased one of their desks a few months ago, and while it's a bit spendy, the quality is fantastic. I expect to be using it 15 years from now.
[0] https://www.upliftdesk.com/desk-accessories/movement-exercis...
I'm curious which IKEA top you used. I also looked at the Autonomous frame, but I don't think it'll work with my existing Karlby top (25 5/8" x 74").
There are some decent options available. The "Autonomous" desk is great, I purchased JUST the legs and use an IKEA top to save money.
Now, I'm looking at the "Treadly" treadmill. Technically, this isn't an "under desk" treadmill, but it's the thinnest treadmill on the market and can be operated without the bars upright. So it's perfect for going under your desk, and then under a bed or a couch when you're not using it.