As far as I'm aware you can still buy a standalone version of 1Password, they just do a very good job of hiding on their website. I'd sugggest contacting their customer support and ask.Seeing the replies (and knowing I need to be better than I currently am) I thought I'd have a look at 1password, but it is yet another subscription service. That is a hard pass for me.
Is there a similar one people can recommend where I can just pay a one off fee, please?
Aha, I did a quick Google and found this: https://1password.onfastspring.com/in-app/1password-7-for-mac
Do be aware that the standalone version may not support all the features found in the subscription version. So my advice is still to contact their customer support and ask what the differences may be (and to check that the link I found is legit).