https://subsoap.itch.io/shadertests
You can do 3D, even very attractive 3D (PBR is even possible) but it’s just not treated as first class in the editor / engine as much as 2D is. Hopefully that will continue to improve even if there is (justifiably) no intention to fully compete with the main 3D focused engines.
1/ How is the support of non-gameplay (but still essential) features:
Many of the popular ones are supported and actively updated. If you are familiar with coding it’s reasonably easy for you to implement an SDK on your own too.
2/ Are there successful (= profitable) mobile games developed with Defold? (except King’s game) If not, why?
Family Island is probably the most successful non-King mobile game made with Defold https://play.google.com/store/apps/details?id=com.MelsoftGames.FamilyIslandFarm&hl=en_US&gl=US 10M+ downloads on Google Play alone.
All in all, do you think Defold offers all the tools/weapons to create professional-level mobile games (in every aspect) for gameplay developers who want to focus on the user experience while being able to easily implement and maintain non-gameplay features?
Short answer: yes! With Defold, you will be able to focus on making your game great and not so much worrying about everything else.
You should try making the same small project with each engine and testing them on your mobile devices to get a real idea of which engine is right for you. People here will of course say Defold is best, but depending on your needs Godot or Unity or even Unreal may be better for you.