I agree that Phokas has perhaps received more than his fair share of criticism (or slander). But making him into another Heraclius, or even Maurice (at least in regards to foreign policy success) is rather difficult.
Phokas seemed to be a rather competent person, as he was chosen to lead the revolt. The problem was that he was a mere centurion (iirc), therefore a foreigner to the Constantinopolitan elite. This might have led some members of the aristocracy or the elite in general to start thinking that they themselves might be a better candidate for the throne than what they would most likely consider an upstart no one. There were already signs that Phokas' rule wouldn't be so easily accepted, as Narseh's revolt in Edessa (which offered Khosrau II the opening he sought in order to start revising the settlement of 591 to his advantage); perhaps (other) conspiracies took place later on, which made Phokas feel increasingly threatened and therefore led him to adopt increasingly harsh measures to quell dissent.
But even if we make Phokas be an almost universally accepted emperor, there are larger forces at play, which Phokas can't deal with effectively, not because he is Phokas, but because almost no one could. The effects of the plague of 541 were still felt, as the empire's tax and manpower base still suffered by the countless deaths caused by this event, and which made it even more difficult for the empire to regain its financial well-being after Justinian's reign. The prestige of the emperor had been steadily declining since 565, due to various factors, the empire was facing constant pressure on almost all fronts and couldn't respond to all these threats effectively, as, again, the plague had irreversibly damaged the empire to a large extent, and was suffering from internal turmoil. In 602, the empire was in a relatively strong but also rather precarious position; a shock like the revolt of the armies of the Danube, the overthrow and the execution of an emperor (which was in many ways the first time something like that would happen in Constantinople) would only serve to destabilise the situation further, meaning that it would become even more difficult keeping all the cats marching straight in the future.
However, we might manage to pull a more competent Phokas (with some luck of course): let's say that Maurice fell very ill shortly after the rebellion broke out and died shortly before Phokas reached Constantinople. Phokas is made emperor, there are still some grudges, but the process is still seen as somewhat legitimate and not as an outright and violent coup. We also gain three things :
- Khosrau II doesn't have the casus belli he was offered accidentally by Phokas when the latter executed Khosrau's "patron" and the man who defied the rather vehement opposition of the Senate in 590 in order to support him in his fight against Bahram. This doesn't mean that Khosrau will be fully ready to accept the new status quo, but it means that he can't move at least in the near future; the treaty of 591 still holds, at least for the time being.
- This in turn means that the Byzantines are able to maintain a large military presence in the Balkans, although there would be a scaling back of the operations there (certainly no winter camps across the Danube). The Constantinopolitan government could therefore be in a better position, at least in the first years, to fend off the Avar raids.
- Narseh most likely doesn't rise up in revolt; this is rather important, for IOTL, Narseh's rebellion could be considered to be the beginning of the problems: it opened Mesopotamia to the Persians, which was the beginning of the Persian penetration of the empire's eastern provinces; it also led to the weakening of the empire's military, as it forced Phokas to first send most of the eastern armies to deal with the uprising (but the Persians managed to ambush the Romans and inflicted them terrible losses - including the magister militum, Germanus) and later on order the transfer of the entire army of Europe to the east, only for this second force, to be ambushed yet again by the Persians in Arxamoun and be almost annihilated.
Therefore, Phokas is offered some respite. If he lowers taxes somewhat and provides the soldiery with better condition and higher pay, then he has most likely secured the support of the army. If he adopts a more moderate policy and instead of reverting to full Chalcedonian Christianity, he makes some moves towards that directions and makes some concessions to the papacy, he might manage to keep most in "controllable discontent", which is infinitely better than continuous strife.
Now, I don't think that this "honeymoon" would last for long; perhaps by 605-606, the Persians would start making some noises. But ITTL, the empire is (far) more stable, the army is far stronger and the empire perhaps still has a couple of rather competent, if not talented, commanders at its service, while a more confident Phokas might not rush to fill all senior positions with members of his family, meaning that we could probably avoid seeing Domentziolus as general and Komentiolus as curopalates - and as a result, have a more effective military and civilian administration. Although this would most likely not be enough to keep everything as it is, we would most likely see a rerun of the reigns of Justinus II and Tiberius II, as the Persians make some gains in Armenia and Mesopotamia but just that and the empire's defences don't crumble and the Avars continue their raids, but apart from that, they do little else. If Phokas manages that and then dies in the 610s (illness, getting too drunk, I don't know), then whoever succeeds him will definitely be in a far better position than the one Heraclius found himself in when he became emperor.
