You could place 1-2-4-5-6 like this:
# | A | B | C | D | E | F | G | H | I |
---|
1 | # | # | # | . | . | . | # | # | # |
2 | # | # | # | . | . | . | # | # | # |
3 | # | # | # | . | 5 | . | # | # | # |
4 | . | . | . | . | . | . | . | 1 | . |
5 | 2 | . | . | 6 | . | . | . | . | . |
6 | . | . | . | . | . | . | . | . | . |
7 | # | # | # | . | 4 | . | # | # | # |
8 | # | # | # | . | . | . | # | # | # |
9 | # | # | # | . | . | . | # | # | # |
Similarly to Donk, but I think slightly better.
1) You wanted 6-es to be able to make a full chain. If you place them outside of the middle you have a good possibility it will end up as the inner minor chain, which usually only covers 4 areas. So therefore I put the 6 in the middle to make it the inner major chain.
2) I put the 1 on the correct side, Donk on the incorrect side. We both have a number in D5, the inner major chain. If the inner major chain is going to be D5-E4, you can place 3 1s in the left zone: A4-B6-C4. If the inner major chain is going to be D5-E6, you can place 3 1s in the left area like this: A6-B4-C6. So it is better not to place 1s in the left area and thereby committing. Place the 2 in A5 and you can later decide how to place the 1s.
3) If my outer major chain is going to be 5s, I want to place a 1 in H4. If my outer major chain is going to be 4s, I want to place my 1 in H6. That would allow me to place 3 1s in the right zone for the occasion that 6s do not complete (allowing me to place a 1 in G4 or G6). I have to guess here and since I prefer to make my outer chain 5s, I place the 1 in H4.
Now I don´t like the guess I have to make in 3). So the actual correct solution imo is this:
# | A | B | C | D | E | F | G | H | I |
---|
1 | # | # | # | . | 2 | . | # | # | # |
2 | # | # | # | . | . | . | # | # | # |
3 | # | # | # | . | . | . | # | # | # |
4 | . | . | . | . | . | . | . | 1 | . |
5 | . | . | 5 | 6 | . | . | . | . | . |
6 | . | . | . | . | . | . | . | . | . |
7 | # | # | # | . | 4 | . | # | # | # |
8 | # | # | # | . | . | . | # | # | # |
9 | # | # | # | . | . | . | # | # | # |
This maintains the flexibility of making 4s or 5s the outer major chain, but whatever happens, the inner major chain can be made D5-E6, making H4 the correct square for the 1 in the right area, allowing 3 1s to be placed there in case 6s do not complete (alongside the 3 1s that can be placed in the left area).
Note that the 4s and 5s can be switched. Note that placing the 5 in G5 (not C5) is inferior, because if the outer major chain is going to be 4s, you will have a badly placed 6.
Final note: placing the 2 in D2 (or F2) and not E1 is not bad and may even be slightly better. It allows you to better handle a 2-flood, but also has its risks, because you need another 2 in F2 that may not come. A 2 on the side has always to be complemented with a 2 on the other side. If you place only one 2, it should be at the bottom (to allow place for two 1s). A lone 2 on a side allows only space for 3 bonus enhancing numbers (2 2s and a single 1).