Try upgrading your brake fluid first, Castrol SRF seems to be a favorite on this board for performance plus longevity. It was one of the best, inexpensive upgrades I did for my car. Some users have reported ss brake lines are an upgrade for appearance (and perhaps durability), but may not be necessary for actual braking performance. The difficulty is brake fluid is replaced and often upgraded at the time of ss brake line installation, so users who report their upgrade to ss lines resulted in brake pedal firmness improvement may actually be experiencing the benefits of fresh or upgraded brake fluid and may inadvertently giving too much credit to the ss brake lines.
Here is a
Castrol SRF discussion link for a few opinions.
And here are a few posts from members who reported the ss brake line upgrade may be unnecessary to gain a firm pedal:
CanAutM3
GroundPilot
admranger