Traveling thailand for 3 weeks

Hello everyone,

I‘m planning on exploring thailand for three weeks with my boyfriend. Has anyone any tips where to go? I thought about landing in Bangkok, traveling up to chiang rai/mai, then flying down to ko samui/ko phangan, then I thought about taking the bus to krabi and in the end flying back home from Phuket. A lot of people would fly back up to Bangkok and take a flight home from there but I don’t really get why because you could save yourself money if you would just fly back home from Phuket. Is that a good travel route to stick to or does anyone has any other suggestions for me?