It's vitally important you know what a MWBC is and how to recognize one, when you are doing things with existing circuits and quads/2-poles. Placing both legs of an MWBC on the same pole Is Very Bad and can start a house fire.
That said, yes, 2-pole breakers in general can be used for two independent circuits.
However, in renovations you typically need to build to modern Code. It's very common for people to do a reno and go "Aw, I'll just re-task the original wires that came here". Nuh-uh. Your kitchen reno needs to comply with modern Code, and that includes wiring, and that includes AFCI/GFCI breakers as required. And where AFCI/GFCI breakers are required, that precludes use of double-stuff breakers. Which is going to be a problem in a tight panel. Code says "too bad; upsize or subpanel".
And yeah, that may be a good idea. If you can find a suitable mounting point (e.g. facing a pathway so that nobody ever blocks the working space), adding a subpanel to the kitchen reno makes a lot of sense. A well-powered kitchen needs 8 circuits or so. That lets you remove all existing kitchen circuits from the overstuffed main panel and substitute with just 1 2-pole.
Now, we strongly recommend wildly oversizing any panel; so if you just heard "8-space panel" what we meant you to hear was "30-space panel". Spaces are dirt cheap when you're buying the panel. And they save you a fortune later when you need to add spaces.
Not lightly do I spend your money, but please - spend $60 extra on another 20-30 spaces. It's the best investment you can make in electrical, bar none.