DeGoogle Yourself

If I just upgrade to the latest version (by flashing) and not install mind the gapps, is it basically a degoogled phone?

It would probably have to be a clean install that wipes userdata, not an upgrade. But yes, LineageOS is pretty much as degoogled as you can get on an OS forked from Google's AOSIP, if you don't explicitly add GApps.

German privacy researcher Mike Kuketz does not agree, but it all depends on how much effort you're willing to put into degoogling, or how much you're willing to compromise on usability.

If we're talking about standard LineageOS, without gapps, it will lack basic tools that some apps need to function. If you want Lineage without gapps, your best bet is LineageOS for MicroG which replaces various Google backend components with free alternatives.

/e/OS does this by default, and what also mainly sets e apart is that it also includes various Nextcloud-based cloud services as alternatives to Google's cloud services. If you don't want e, these tools can still all be setup yourself, though with less convenience.

Personally I did not have a good experience on /e/OS. But I'm not so sure that was a fault of the system itself. At the time I was using a Teracube 2e, which didn't have a good user experience to begin with.

If you have a compatible device, I've found Iode to also be nice. Any of these options would be much better than stock Google malware.

To answer your technical question with perfect accuracy:

I don't know.

To answer your privacy question:

Yes. You'll get roughly the same privacy benefit.

Standard disclaimer: to gain full privacy from Google also requires regular use of a VPN, strict 3rd party cookie blocking, regular clearing of any mystery cookies that show up, and some luck.

You might be missing some of the advance privacy features. I'm not sure... I've been on e/os for a few years now and haven't checked out LOS recently. Have a read of this

I thought eOS had some apps that were only specific to that distro, so they would be missing from LOS