There are fierce regional rivalries, but as is often the case in civil wars, you don't get to choose your side, your side is often chosen for you. The Oromo Liberation Army's goal is for an Oromo nation-state with it's capital in Addis Ababa, renamed Finfinne. For Amhara this is a gun pointed at their head, and victory would lead to the biggest refugee crisis since the 1950s.
Abiy has devoted very few resources to combatting the OLA and has instead focused all his efforts on fighting Tigray, who have no territorial aspirations over Amhara territory, but Amhara have territorial aspirations over Tigray territory. This has led to the worst case scenario for Amhara people, where they have pursued a genocide of Tigray to seize claimed territories and have thus forged an alliance between Tigray and Oromo, one seeing Amhara as an existential threat to their own life and lands, and the other who seek the land of Amhara.
I suspect Amhara, if they had been able to navigate their own course, would not have taken this dreadful path for themselves to follow, but the 2019 coup removed all options from them, they instead follow the course Abiy has laid out for them, the course towards destruction.
The best option for Amhara right now honestly is to surrender to the Tigray Defense Forces and allow Tigray to arm and train an Amhara force to occupy Amhara lands. This would give TDF an incentive to turn over any captured territories to their Amhara proxy. At the moment all incentives are to turn over any territory they can to their allies in the Oromo Liberation Army or their Agaw Liberation Front catspaw. Remaining loyal to Abiy means going down with the ship.