Frank not showing the iban on a crawlable place might be safer
There's not really a way around this afaik. Because of regulations, the person using a bunq.me page to pay someone needs to have the possibility to see the IBAN. Not sure what you mean with "non-crawlable" but as long as it's visible in a browser, you'll gonna be able to crawl it.
non guessable link would be safer
Yes, that's right. At least for people who don't want to have their name + IBAN combination public. Could be a good idea to have some kind of option when creating a bunq.me page that lets you choose between "random string (safer)" and "custom name" with a short explanation that a custom name should be considered as public.
There's not really much someone can do with a bunq IBAN, as direct debits always have to be confirmed up front but for users that aren't so aware about social engineering tricks it's indeed probably safer for them to have this private. Gets a hi-five from me!