An enterprising person over here has provided a translation table for the letters and some of the numbers, in multiple typefaces:
Someone somewhere may have put together a more complete version; this version is from just after episode 3 or 4, and more sourcetext became available as the show went on.
As you surmise, there is a simple one-to-one relationship between the English alphabet (and related glyphs like punctuation and numbers) and the KonoSuba alphabet. However, beware: the actual text that is enciphered is not necessarily in the English language - some of it is also in romanized Japanese. For example, someone in the linked thread deciphered this part of your image:
Which may seem like gibberish, but in (somewhat typo-laden) Japanese, it's Megumin babbling about EXPLOSION. Roughly, "A wunderkind magician, even moreso than the average Crimson Demon. She is enraptured by the mightiest of magicks: EXPLOSION. She won't use any other magic, and plus - she can't." This particular text is cobbled together from various bits of copy about her character, as far as I know.