You’ve gotten a lot of advice, so this might get drowned out: you cannot replace AFCI or GFCI breakers with tandem ones that are not AF/GF. You have 4 single pole non-AF/GF breakers throughout the panel that you can replace with tandems. Since they are not next to each other it would require you to move around many breakers in order to get a 2 pole space. Not a big deal, but it will be the last circuit you ever add. Your best bet is to install a sub panel right next to your main panel, if you have room on the wall.