From 3.5 to 5.6.2 – Recommendation on how to get there with the least amount of pain?

I’ve recently been asked to help someone who’s neglected their WordPress for a long time. They’re currently on 3.5. I need to get them current. I’m considering the following options:

1. Manually upgrade step-by-step. Outside of the pain of more than 20 steps, I’ll also likely need to upgrade plugins when the old version no longer works with the next WP version.
2. Export the site contents using the WP export, then re-import the content. I’ve got a few plugins that won’t export (and I’m not sure I want to deal with import/export of them from the database directly).

Any advice before I spend more time than I’d think it’s going to take?

Considering that i) as @Tony Djukic put, “depends on how complicated the site is and how much of it is worth keeping around” ii) pain is something without an objective way of measuring iii) there will be some pain (and you acknowledge it). I say: just rip the Bandaid and go with option 2.

Evaluate if the website really needs the features and functionality the plugins provide. If so, add them progressively later. If the side doesn’t need the features it has today, you just got done and have a clean(ish) slate, and it may even be less painful than you thought.