Guillaume I think you are confusion security (protection against unauthorized access) and privacy (control over what data you share and with whom). I agree Apple (and Google too btw.) are about as secure as a big tech company can get. They deserve all the praise they get!
Regarding privacy, Apple just keeps failing in my opinion while performing great privacy theater that in the end is not effective (when it comes to privacy towards Apple). I'd go as far as to say you don't even own your iPhone as Apple has more privileged access to it and your data on it than you.
I feel like I'm derailing this thread now. It shouldn't be about whether Apple is private or not, but about OP's question whether he can gain privacy towards Apple by only moving away from the iPhone but not the other Apple equipment.
csb146 So am I gaining anything from a privacy stand point ditching my iphone if I'm still tethered to the IOS ecosystem at home?
I believe any step in the direction is a good step and you'll definitely gain privacy towards Apple by just replacing the iPhone. It is the most sophisticated data collection device of the bunch with all the sensors, apps and use cases. Also you'll have it on you at all times while most of your other devices are stationary at home.
That being said, this step will also be the hardest as Apple has put a lot of effort into making the use of their devices as seamless and comfortable with the iPhone but unnecessarily convoluted with other devices. A Pixel with GOS will not work as well with your Macbook, HomePod, AirPods, AirTags etc. but you can make it work somehow.
If you decide to go down that path, I suggest you keep your iPhone around for a while to have an easy fallback solution if a task is too annoying at first. And you'll have an easy exit in case you end up seeing not enough value in using GOS for all the inconvenience you are having in the Apple ecosystem.
The good news are: It's likely you start to enjoy all the privacy features you never had access to on your iPhone. You might get annoyed by Apple for restricting you this much and slowly replace your accessories piece by piece and end up with the privacy that is fitting your threat model. Take it slowly and don't burn bridges just yet. Privacy is best achieved when you take your time and make educated and informed decisions along the way.