You diagnosed well.

Underproofing is the major factor though. Gotta go more by volume change than time. That's true of packaged yeast as well, but that tends to overproof rather than under.

You said you cut too early as well, which I suspect is the under baked part, rather than not enough time in oven. The crust looks maybe a tad under, but a lot of that comes down to personal preferences, and it does look like the time in the oven was within the range to have gotten the job done. But the carryover heat is what finishes a bake inside. Well, usually.

Combo recipes like that aren't unusual though. A lot of sourdough sandwich bread adds packaged yeast. Some desserts like cinnamon rolls as well. I don't think it's really necessary, as the results are essentially the same, they just take longer without the packaged yeast. You might see a tiny bit softer crumb with some recipes by adding regular yeast, but I've never noticed a difference there.

What you will get is a milder flavor. The longer proof of pure sourdough gives more development time, and more tanginess. I can't say it's a huge difference, but it's more noticeable than the texture