Welcome to the party!
First, I'd like to reassure you that yes, this is not only possible, it's easier than many would have you believe. I can't even count how many "Durrr, I tried ditching Google and durrr it impossible durr" opinion pieces and YT vids I have seen.
I suggest moving things away from G in steps. Have you familiarized yourself with the GrapheneOS profiles feature? You could do what many here do and create a profile specifically for your Gapps and only have the limited GPlay and Play Store in THAT profile.
Then in another profile, you can only download FLOSS apps only. There are several ways to go about this each with their own ups and downs, so to speak. For instance, you can make this "open source only" profile the MAIN phone profile and just install within it....or....you can make it a sub-profile justt like your Google profile. Many tutorials within the forum here on setting this up...the "search" function here works quite well.
Also, where you get your apps is of importance, naturally. Using Droidify (can be DL'd from apt.izzysoft.de) is going to be the easiest and is plenty safe, IMO. Droidify just acts as a skin to access various repos such as F-Droid's. I highly recomment you enable at least the IzzyOnSoft/IzzyOnDroid repos but also check out the others listed and even more that aren't.
As for apps: IMHO, I'd start by moving away from Gmail. Personally, I use Tutanota. The devs have been very good to me over the years, they have a couple different paid tiers with great value, they are respected/trusted within the "privacy/security" community... And as a bounus, if you choose @tuta.io (they have several domains including .com to choose from) as your email domain, you get to hear the gears turning in the person's head every time to you tell your address. "What? Dot eye - oh? No dot com? Is that right? Are you sure??" lol
Again, with Tuta (or whomever you go with) in one profile and Gmail still working when you need it in the other, GOOGLE profile...this makes the transition much easier with email. You can migrate as quickly or as slowly as needed and be sure you get everything switched before ditching your G profile for good.
Same for other apps, I suppose. This was my method of leaving G. I still have my G profile, NGL. But I no longer need it nor do I even utilize a G profile at this point. All FLOSS, all GOS. It can be done.
I'm sure others will chime in with more and the GOS site has some great tutorials to touch on your other questions. Good luck and have fun! You are really going to be impressed with this OS.
EDIT ADD: I still have my G *account but no longer use a G *profile on my phone...is what I meant to say. :)