Hey OP, I feel you as I've been in similar situations recently, a few things that have helped me protect (aging) loved ones on the internet:

- uBlock Origin [0] in every single browser in every single computer in the household. This is non-negotiable in my opinion. It's the single best deterrent of scams and malware you can set up for anyone.

- NextDNS [1] as the router DNS, as the system DNS for every device, and as the DNS for every browser. This allows you to control more blocklists remotely for your father if he finds any issues. It also provides dynamic DNS-level blocking through AI and heuristics, along with the usual blocklists.

- Not used to macOS, but you likely can set up the user account to not be able to install applications (i.e. no root), this should help a lot. Using macOS or Linux is a huge win in security, simply due to distribution repositories or the app store being way more secure than downloading random EXE files from the internet.

- You will not manage to get him to use a password manager, or at least won't get him to use one correctly, so set up SMS 2FA in all of their accounts. "SMS? Isn't that insecure?", well the truth is that they will have awful passwords, you won't be able to change that, and they won't bother TOTP codes, so SMS 2FA is the next best thing. SIM swaps shouldn't be in their threat model.

-- Regarding password managers, you may be able to set one up to autofill, change password to generated ones, set up TOTP in the password manager. However, the big thing is to not expect your father or anyone else to actually bother with the password manager when creating new accounts. If you do set it up this way, tell them about it, educate them on how to use it, but make sure to nail on your father's head to NEVER type a password that's in the password manager, and instead always rely on autofill. If you're not sure which password manager to set up, take a look at Bitwarden [2] (FOSS) or 1Password [3] (which I hear is very simple for "normal people").

- Regarding credits cards, tell them to set up strict spending limits. I'm not in the US, so I don't know how's the situation there regarding virtual credits cards, but I personally choose to create a new credit card for every purchase I do. If your father has any subscriptions they have to pay, you could help him set up these virtual credit cards and assigning them to different services. Don't even take note of the password, the only use they should have is to be cancelled, you shouldn't use them to spend on anything else.

- Regarding phone security, set up caller ID and maybe even block unknown callers. For caller ID I personally have them set up with a Samsung phone which ships it by default, I'm also aware in the US some carriers may provide that service to you for the landline too apart from an app in the smartphone.

- Last, but not least, set up an email client or email service which is excellent when it comes to blocking spam. Gmail ain't it, unfortunately. I can't give you many pointers regarding this though because I'm not sure how often email will be needed nor what applications exist for macOS email clients, so you'll to search more on this.

Overall, these are the tips I'd give, so you can get started. Be aware this won't solve all issues, but it should make your life and your father life orders of magnitude easier. Best of luck and godspeed.

[0]: https://ublockorigin.com/

[1]: https://nextdns.io/

[2]: https://bitwarden.com/

[3]: https://1password.com/