Hard one! Where to even start?

No logical start -> bad
multiple answers possible -> bad.

Spoilers/Hints
Corners sort easily, then 1 is in the middle of R1 and R20, then 1's diagonally (left and right) down from top and up from bottom. C6 blocks start R6 and R12, C16 blocks start R6 and R13, C11 blocks start at R5 and R13. Join them all up.

Thanks for the hints, CloudyDay :-) But the "middle" of R1 and R20 is in C11 ;-)

Finally! Without hints impossible to do, and even with them very hard = very bad :/.

How anyone rated this more than 1/10 I'll never understand.

Terrible as a hanjie and unaesthetic as an image.