2.5 gallon batches are done with 4 gallons of water in a 6 gallon kettle. I’ll lose 2-3 degrees over the course of an hour mash.
That's not too bad, better than average I'd say. Any insulation around it and on top of the lid during the hour mash?
I doubt the 6 gallon kettle would fit inside your oven, so yes, some extra insulation would be helpful.
If you take the kettle off the stove during the mash, don't forget to put some insulation under the bottom. Putting it on a cold floor or countertop is a huge heat drain. And yes, you'd also lose the residual heat from the element the first 15-30 minutes. After that, the glass top likely becomes a heat drain itself.
1.5 gallon batches are done with just over 2 gallons in a 4 gallon kettle. 3-4 degrees down is the usual.
That one may fit inside your oven. I used to keep mash pots in the prewarmed oven when I did partial (30-50% all-grain) mashes. I would turn the oven on for 2-3 minutes half way through, and the mash temp was usually within 1-2°F of the target during the hour.
I like my 54 qt converted cooler for 5.5 (and the occasional 11) gallon batches, but it's tricky and it took some time and diligence to hit the mash temp right on the nose, erring in coming out a bit higher rather than being 1-2 degrees lower. It's much easier to cool it down by adding some cold water after stirring, than trying to raise the temps, even 2-4 degrees.
I still sometimes mash in the kettle (step and decoction mashes), and like having the benefit of being able to raise the temp by just turning the heat on for a few minutes while good stirring/bringing the bottom of the grist to the top, while scraping the bottom well to prevent scorching.
BTW, I also brew in the kitchen, using a 3500W induction plate on the countertop. Having the stove nearby is handy for boiling down extra wort and such.