Building some apps on the AppStore since more than 5 years now, I get around 20k installs per year and sell for around 3k€/month thanks to multiple iOS apps.

The new Addendum for apps in the EU seems to offer better conditions (lower fees mainly) BUT I'm afraid to accept without understanding some negative impact that can hurt these apps.

For an indie dev, would you recommend him/her to accept this addendum? And why?

For now i would wait. The rules are currently changing to fast. Epic got thrown out and hours later their account was back again. Many people expect the EU to fine apple, since they think apple uses a tactic called malicous compliance(follow the rules to the letter but not a millimeter farther).