I believe that Apple’s iCloud Private Relay uses Cloudflare and Akamai servers to run the service this is also something to consider. IP addresses are only one data point that advertisers use to track people, I personally believe that using a free vpn (like Proton VPN) and using a privacy conscious browser like Brave, Firefox or Mullvad Browser will offer better web tracking protection than using Safari with Apples Private Relay on macOS. Advertisers are moving further and further away from tracking users using IP address.
I think your best (and most cost effective) option if you don’t want your ISP to collect your website history is to use iCloud Private Relay on your iPhone and use free Proton VPN on your macOS computer along with a privacy conscious browser.