The most private recommendation is either no anti-virus or using only built in security tools.
A Stock Samsung already has McAffee and Google Play Protect.
Google’s Advanced Protection makes it far harder to hack into you account. At the surface it requires a Passkey to log in to your account but it also enables and enforces stricter security settings.
But yes, a government can still force Google to share information connected to your account.
Samsung’s Auto Blocker replicates features from Apple Lockdown and Google is currently expanding it’s Advanced Protection Program to do so as well.
Currently Apple Lockdown is the only tool that’s consistently proven to protect against the sort of targeted malware attacks governments have access to.
They’re far from the only thing to enable if you’re being targeted by a government but they’re some of the most robust options available aimed at making targeted attacks more difficult.
I don’t personally use custom ROM’s. Graphene OS on a Google Pixel is the only one I’d be willing to try but Pixels have no official support in my country (Grey-market import) and the cost is essentially the same as a comparable iPhone, where they seem to be a lot cheaper in the US.
Samsung also tends to have little support for custom ROM’s of any kind.
But if you do plan on looking into it.
I’d strictly recommend getting a reputable ROM from an official website after confirming compatibility with that website. XDA forums and such do exist, but they can also be a malware vector. No amount of anti-virus can protect an OS from embedded malware.
When looking at compatibility check the Model Name in About and compare that as well if you can because some models have incompatible variants.
Graphene OS and Calyx OS are by far the best ROM recommendations. LineageOS is okay, but mostly aimed at extending support, not de-googling or better security. I’m unclear on e/OS but it seems to support a similar model list to Lineage based on a quick look.
Only Lineage and e/OS have support for any Samsung models.