The problem with the 3rd Century is a lot of it was essentially random politics and battles potentially starting usurpations and civil wars and there is always going to be friction thanks to there being 2/3 of the army needed to defend the entire Imperial border so whenever shit goes down somewhere somewhere else will likely see a raid or full scale invasion whilst the troops are away.
Yes, as we already discussed elsewhere one major reason of the 3rd century crisis was the multifront war. And a single roman princeps could just be at one front at a given point of time. This (amongst other reasons) led to usurpations in the defenseless provinces.
But with his co-emperor Gallienus and the capable and rather independently acting Odeonathus, Valerius could have covered all 3 major fronts (Rhine, Danube, Euphrat). A kind of tetrarchy without multiple emperors and without a division of the empire. Of course more measures were needed, but Gallienus was not that bad in introducing reforms. At least, there is a chance, that the principate recovers and changes into the right direction. A very late POD to change the empires fate, but perhaps not too late.
And I do not think, that you really need 2/3 of the entire army to repel an invader. Postumus in Gallia did very well and so did Zenobia in the East. And both had not 2/3 of the army. Actually no roman army ever was 200.000 men strong. The exercitus of a princeps on campaign was rather 60.000-100.000 men strong. It is rather a question of strong government and onsite presence of powerful representatives of the emperor onsite and trust into the capability of the emperor and his guys to secure the borders.