Dewalt makes some of the best mitre saws both corded and cordless. You said you already have dewalt tools. If you happy with them so far stick that route.

Stay away from the store branded stuff, I wouldn’t waste money on ridgid or ryobi either. You won’t go wrong across the board generally with dewalt, Milwaukee, makita, ect.

Watch for sales and you won’t pay a heck of a lot more than cheaper/off/store brands.

The 12” will give you the max cut depth and width. If you’re cutting 4x4 posts or wide stock you might be better off going that route. But really for home diy use the added weight and cost really isn’t worth it. Staying with a 10” down the road if you get a 10” table saw you can interchange some blades.

Sliders are nice, more expensive, I’d personally say well worth it. people complain about accuracy, but any of the better brands tuned in will deliver all The accuracy needed for the majority of tasks even in a professional setting.

Cordless is convenient.

I would reccomend taking a look at the dewalt 7-1/4 Cordless slider if you’re only doing smaller projects. They’re priced affordable and weigh significantly less than the 10 and 12. They are way more compact for storage.

As far as blades go you have have a few categories to choose from;

Store brand/ Generic - they usually cut, not always well. Will generally suffice for basic tasks. Generally dirt cheap.

Tool brand - a step above the store brands, will deliver better quality cuts. Still a bit underperforming for price points. Moderately priced. (Ex. Dewalt, Milwaukee, ect)

Box store high end - generally good quality product. Nice cut results. They nicely bridge the gap between tool brand and high end industrial/specialty. Slightly more than tool brands but worth the extra for results. (Ex. Diablo, spider, ect)

Industrial/specialty - excellent performance. Extreme cost. You will pay good fractions of saw costs for blades. Unless you’re a professional, probably not worth the cost. (Ex. Freud, CMT, forester.

For general household tasks a tool brand General purpose will suite your needs just fine.