Hi Loic,
You ask a tough question because your definition of painless and capable may be different than mine. For me, being capable means carrying a payload for mapping, FPV, video, and longer range flights. I consider it a fun part of the hobby to create a VTOL but also appreciate a design that is easy to convert. These choices all depend on your goals.
The Hobby King Bix3 was a nice plane to start conversions on because it was relatively inexpensive, could carry a light payload, and, flew slow enough to see just what was happening during transitions. It was difficult to stall so mistakes were forgiven. Spare parts were also available.
You’ll need to choose a design that is best for you. Your goals may even change before you finish your first project. If you don’t see anything that fits your needs, then wait a bit and something may arrive sooner than you think.
Good luck!