There are other ways to block ads. Adguard does a great job on Android. It establishes a local VPN, so it can do HTTP[S] content filtering in addition to DNS blocking.
Can't use my VPN and adguard at the same time iirc, unless android has two active VPN "slots" now. Can't bring a pihole with me 24/7 either as much as I would like to.
Yes because there is no need to setup another VPN. You only configure the DNS settings (Private DNS). I know that Mullvad on PC has an option to use custom DNS server