Question for Wine-os!

I agree with ChefJune on mulled white wine: I've never had it and can't imagine it...but good luck if you do. I use either a mid-range pinot noir or a granache for holiday mulled wine and I almost always add brandy.

I like to do more mellow, fruity (not sweet) wines for mulling. I risk sounding like a total bumpkin suggesting boxed wine on a food site of all places, but it's my favorite for mulling - I've used Black Box merlot or Big House zinfandel for larger groups. I've never done white mulled wine, but I would tweak the spice mix so it's not so heavily spiced. But again I'd go for something easy-drinking, and not complex to begin with.

Imho Cabernet Sauvignon is too tannic to make a good mulled wine. I would suggest Beaujolais (NOT Nouveau) or a Grenache/Garnacha based red. Those are light and fruity and lend well to mulled wine.
I really have not seen mulled wine made with white wine, and can't imagine it. But when I'm making Sangria I don't choose Chardonnay, because many of them are oaky, and that doesn't translate well into a fruity mixture. Sauvignon Blanc, Pinot Gris, Pinot Blanc, Gavi - all are better choices for mixing "stuff" with white wine.

Mulling adds so much flavor that wine taste is almost drowned out.
Get decent dry red or white (your pref) that's on sale.
Then you can control sweetness & not worry about wasting a costly fine wine.

Cabernet for red, Chardonnay for white or any wine you enjoy. I'd suggest a slow cooker if making it for a large group.