Is the で in という理由だけで a form of だ or the instrumental particle?

Question

I'm trying to figure out how to say "Maybe it's just because ____, but..." in Japanese, but I'm not sure how to place 'just because' in its own clause that way.

If I'm not mistaken, the expression '____という理由だけで' roughly translates to 'just because ____'. However, since I've only ever seen it used as part of a sentence about something else, like '友達がその本を推薦したという理由だけで読んだ', I can't tell whether the で in it acts as a particle, or the te-form of the copula.

Obviously, this is a problem when I'm trying to use the expression as a clause to add が to in my sentence. Does anyone know the proper way to do so?

Answer 1

The use of で here fits the second Wiktionary definition:

with, by, using: indicating the means by or with which something happens

The で here attaches to 理由, rather than だけ. 'Via' and 'per', and 'for' could also be suitable translations given the right context.

〇〇という理由で　&　〇〇というわけで　For that reason; on the grounds of _____.

I assume you want to add が to mean 'but'. If you show the specific sentence that you have in mind and attempt your own translation a fruitful answer may emerge.