Yes you still do. Sophisticated companies (Amazon, Google, Meta, many others) will serve ads from the same domains that serve the content you actually want. Pihole can't block the ads without blocking everything. Ad blockers are necessary to block specific pieces of content from amazon.com, youtube.com, etc., while still letting you use the service. It's a constant cat and mouse game, but ublock origin does a good job of staying updated.

Probably not. I don't own one as I'm lazy. I actually paid for AdGuard and use it on my iPhone and iPad. It comes with a mini PiHole implementation built in.