I’m fairly new to custom ROMs, having converted my Sony to /e/ in summer 2021. Before that, I always disabled, or attempted to disable, anything and everything related to Google on my Androids, and only used F-Droid (or sometimes Aurora store) for apps. So for me, the difference in experience is minimal, except that I no longer have to worry about being spied on by the operating system and Google Play Services. (/e/ uses micro-G as a replacement).
Note, though, that I’m probably not a typical smartphone user. For example, I would never use Google Maps or other services, I don’t do banking on my phone, I don’t use Google’s YouTube, I don’t use music streaming platforms (except internet radio), I don’t do gaming, and I don’t watch movies on tiny screens.
As always, I stick to open-source alternatives (OSMand+ for maps and navigation, drop-in FOSS replacements for messaging and email, etc.).
The only issue I’ve had with /e/OS is that if I shut down the phone while it’s on 4G, when I power up again I have to switch to 2G or 3G for a few seconds and then re-select 4G. I believe this is a LineageOS “bug,” as it’s the same with my Lineage-loaded devices (both Sonys). If not that, then it must be a Sony-specific issue.
Battery life is drastically improved, and there are way fewer background processes running (for my configuration). For a FOSS dévoté, the experience is refreshing and awesome. For someone who uses Google-dependent apps, probably not so much.
EDIT: One other issue. I found that MMS doesn’t work unless I use my mobile network’s DNS resolver. Trying to use a different DNS provider prevents MMS (and therefore, group messages) from sending or receiving. That problem may be specific to my mobile provider, though. I’m not sure. (And it would probably be the same with standard Android.)