Especially considering they fought arguably more formidable opponents than in the time of Augustus or the five good emperors”. I’m interested in regards to post Constantine emperors.
Where do you think these emperors are overlooked? You might just not be looking in the right areas. The study of the later Roman emperors is huge. The best reference book for everyone in the late Roman Empire is by A. H. M. Jones and J. R. Martindale, The Prosopography of the Later Roman Empire, Vol. I: AD260-395. A. H. M. Jones' other main work, The Later Roman Empire, 284–602: a Social, Economic and Administrative Survey is also a great place to start. The online encyclopedia, De Imperatoribus Romanis, is also a very reliable resource with an extensive bibliography for all the emperors. Most late Roman emperors have at least one bibliographic monograph dedicated to them, although some are becoming dated.
I should specify its western roman post Constantine emperors like Valentian I or majorian.