Best not to make sweeping generalizations of seniors. Sure, there are some that have lost mental faculties and can barely see from cataracts, but there are plenty that are capable of using a normal smartphone. Grapheneos isn't inherently more challenging than another smartphone and most are already using one.
The best advice I would have is to get them to setup the phone themselves with you. And then just point them toward recommended apps. This means spending time with them and having patience.
A lot of the scam prone seniors are only like that because they haven't been taught best practices. They're not always dumb.
When I explain concepts, they fully understand it and demonstrate it with on-point questions and remarks. Don't just do monkey see, monkey do. Be a teacher. Explain concepts, demonstrate how it works, have them try it. And stay organized. Create a lesson plan. You'd be surprised how many small things you do on the regular that you don't think about end up needing to be taught to people. I've met some that didn't realize you can rearrange apps, didn't know how to close apps, how to copy and paste, etc. This is the stuff they struggled with the most, not the privacy and security features.
In many ways, teaching privacy/security to seniors is a lot easier than teaching young people. They have patience. They ask questions. They're not ADHD'd brain rot looking for a quick magic fix in 15 seconds. You're also not trying to unlearn phone and social media addiction to someone who has never known life without it. They're also usually retired, so there's no "I need [random spyware app] for work."
If they have absolutely zero interest, then it's going to be a tough sell. But that attitude is not age dependent.
I just recently explained passkeys to a couple in their early 80s. They had heard of them under the lense of being this annoying thing websites are bothering them about followed by some misinformation they heard through the grapevine. Once I explained how it worked (general concept of public-private key pairs) and then demonstrated how to create and use them, they immediately "got it" and said they wished more websites used them. They asked all the right questions. Without me even prompting them, they even questioned why websites are using them as a password alternative or as a 2fa method instead of as a password replacement since that defeats a lot of the benefits of passkeys (a realization that makes them more competent than many professionals)!
Seniors are also not the same seniors from 20 or even 10 years ago. They almost all have computer familiarity. Someone in their 80s was born in the 1940s. Meaning, they were still in traditional working years during the dot com bubble and even into the 2000s. You're not talking to people that have never owned a phone or computer before.