It may not be easy to grow adoption (and network effects) during the growth phase by having a constant supply and then pumping the demand. People would buy the token mostly for one reason, and that is expectation of appreciation of value. Once the issuer then announces that the growth phase is over, I suspect holders of SagaCoin to sell it. The purchasing power would then drop. Maybe this is OK if the final target value is a stable fraction of the maximum value achieved.

In the case of Bitcoin, there is no maximum target value, so the expectation is that its value would continue to go up. If people knew that Sagacoin has a growth phase in which it is expected to grow in value, and then a stable phase, people will stop buying it once they hear even rumors that its growth phase is almost done. Adoption during the growth phase would therefore be limited.

In contrast, a stablecoin backed by any central bank money can grow much faster in adoption simply because it is a more usable version of that central bank money. People trust this kind of stablecoin more readily because they can exchange it or redeem it for fiat more readily. (I call this phenomenon "Value Trust Transference" - people naturally transfer their trust in fiat to the new stablecoin.)

I think it's better to keep the same promise during the growth phase the same as in the stable phase. When a volatile token that holds the promise of appreciation suddenly changes its character to that of a medium of exchange, it may be difficult to convince the market that henceforth, it will be stable.

A stablecoin backed by fiat can dissociate from it once that fiat starts inflating drastically. I understand this won't be easy (for fiat-backed stablecoins like USDT); however, there are algorithmic stablecoins that seem to be highly stable, like FRAX (see: https://frax.finance/), that can remain stable as it dissociates itself from fiat.