There is nothing wrong with davening with Reform Jews...
Yes there is. In a Deform synagogue, the deform "rabbi" is likely to be mechallel shabbos, even during the ceremony itself. How can you join such a prayer service? It's wrong.
They changed the prayers to English and in some cases changed the words. Do they even lein the entire Torah portion? Do they even have shemoneh esray? Many of these places probably don't even have that. I don't believe it is ok to daven in a reform so-called synagogue unless there is some extraordinary circumstance requiring it.
You mention your past how there was orthodox kiruv at your reform place (they infiltrated it!), and that's great, but now that you know better, you "graduated" to a real synagogue with real davening. (and anyone who knows better has to "graduate" in this way!)