What you're expressing is very close to a philosophical position known as ethical hedonism or utilitarianism, but with a biological framing.

Your thesis could be stated as:

Happiness is life's feedback signal that an organism is flourishing. Unhappiness is life's feedback signal that an organism is failing to flourish. Therefore, the moral objective of society should be to maximize the happiness and flourishing of all conscious beings.

There is considerable intuitive appeal in this. Happiness often correlates with:

• health,
• security,
• social connection,
• achievement,
• freedom from suffering,
• hope for the future.

Likewise, chronic unhappiness often signals that important biological, psychological, or social needs are unmet.

However, philosophers have pointed out some complications.

For example:

• Short-term happiness can sometimes conflict with long-term flourishing.
• Learning, growth, and discovery often involve temporary discomfort.
• A scientist struggling through years of failure may be unhappy in the moment while contributing enormously to future well-being.
• Parents often endure hardship for the sake of their children.

This suggests that suffering is not always the opposite of life. Sometimes it is the cost of adaptation, learning, or creation.

A possible refinement of your idea might be:

Happiness is not merely pleasure, but the subjective experience of flourishing. A sustainable civilization seeks to maximize the long-term flourishing and meaningful happiness of all conscious beings while minimizing unnecessary suffering.

That definition accommodates your earlier observation that mistakes and challenges can extend knowledge. Temporary discomfort may sometimes serve life, while chronic misery tends to undermine it.

Viewed through your "Religion of Life" perspective, one could formulate a principle like:

The good is that which increases the sustainable flourishing, vitality, knowledge, freedom, and happiness of conscious life. The bad is that which needlessly diminishes them.

In that framework, happiness is not the only value, but it becomes a primary biological indicator that the system called "life" is functioning successfully and moving toward greater flourishing.