Which MySQL server parameters should be adjusted to improve INSERT performance for a large MyISAM table?
A long list of possible improvements:

Turn off updating "atime" on the filesystem?

INDEX(id1) can be DROPped since there is another index starting with that.  This would decrease from 6 to 5 the number of indexes to update -- a small speedup.

Toss any other indexes you are not using.

Use NOT NULL where appropriate.  I doubt if you meant for all columns to be NULLable?

With a 1G key_buffer, I hope that you have at least 6G of RAM.  If not, decrease that setting.

Increasing the key_buffer will not instantly improve the index caching -- it takes time for blocks to get into the key_buffer and become useful.

If practical, "batch" the inserts.  That is,

INSERT INTO mytable (...) VALUES (...), (...), ...;


Optimal is to batch 100-1000 at a time.

Switch to InnoDB.

What will you do with the data?  (Usually the performance problems occur on the read side.)

Two Hardware suggestions:  SSD; RAID-striping with write buffer.