[mothered]

First one for PayPal:
When do they get suspicious?

Simply hitting too many disputes/claims, particularly In close timing. 

but what about 'normal' refunds, where the seller agrees with a refund? Might PayPal ask some question why e.g. the last 5 orders where all refunded?

As per the latter part of the above.

Use common sense and good judgment. If you refunded 5 orders In 5 years, you'll obviously be fine. On the other hand, 5 refunds In so many days, will attract attention.

05.09.2021 - DNA'd Amazon (make some legit orders and wait some time before using DNA again!)
10.09.2021 - Boxed Amazon
12.09.2021 - Used Sealed Box

Irrespective of alternating between methods, three claims on the same account In precisely one week, speaks for Itself.