Have Emperor Maurice decide at some point in the 590s to disband large numbers of troops on the Persian border to save money, trusting on good relations with Khosrau II who owe his throne to Maurice's support.
Phocas still rebels against Maurice in 602, and still overthrows him. Khosrau still declares war on Phocas using Khosrau's debt to Maurice as a casus belli, but with the Persians facing considerably smaller Roman forces due to Maurice's cuts to the army. The war goes even worse for the Romans than it did IOTL. The Jewish and Egyptian revolts of 614-616 happen ahead of schedule, as the Persians are in a position to support them sooner, and discontent is higher due to Phocas needing to raise taxes in order to rapidly recruit troops to replace the armies disbanded by Maurice or destroyed or captured by the Persians. Likewise, the Avars invade the Balkans sooner as the Roman forces collapse.
IOTL, the Exarch of Africa (Heraclius the Elder) and his son Heraclius the Younger rebelled against Phocas in 608, Heraclius the Younger took the throne, but by 618 was seriously considering abandoning Constantinople as militarily untenable. ITTL, the Roman are already in the OTL 618 position by the 608 and there's a Persian army in Egypt that could threaten the Exarchate of Africa, so Heraclius decides instead to declare independence and make a separate peace with the Persians.
The Exarch of Rome also secedes. The Avars sack Constantinople and go home. Persia winds up conquering Anatolia and setting up vassal states in Egypt, Syria, and Judea, leaving a rump Byzantine state in Greece to rebuild Constantinople.
Islam still arises on schedule in the late 620s, but face a unified and healthy Persian Empire that's had a chance to consolidate its conquests, rather than the OTL devastated and war-weary Roman and Persian Empires that had just made peace with each other out of exhaustion from decades of war. Persia turns back Muslim attempts to invade Mesopotamia and Syria, but the Muslims still manage to conquer the vassal states in Judea and Egypt, and eventually make peace with Persia. The Exarchate of Africa is a tempting next target for the Muslim Empire, but with fewer resources to draw on and with an intact Persian Empire on their flank, they're never able to commit enough resources to conquer the Exarchate.