Another from (religiously observant) RN here, haha. I hear you. There was a thread a while back where I gave a run-down of what being in the hospital on shabbat/chagim (holy days) looks like. It came up for me when I was precepting/orienting and had no control over my schedule.
I had a LONG conversation with my Rav, who called HIS Rav for advice on how to pasken (make decisions about/within Jewish law) for me (his wife is a pediatrician so he gets it - his outlook has always been "you're doing a good thing and let's figure out ways to minimize the damage" haha. Yes, there are issues - logistical mostly as I had to get there and back well outside of walking distance - but I totally understand that it's hard to reconcile. It also made a difference that it wasn't long-term but was just while on orientation.
What's hard to explain to people (both here and in person) with little to no experience or exposure (and trust me, I get that -- giyoret (convert) here who had no idea pre-20s what a Jew even was, let alone an observant one!) is how intricate it gets and the minute details to think about. For anyone interested in some of the minutiae, when I was at the hospital on shabbat, I couldn't badge into the break room (because it would be using electronics not directly related to patient care). I didn't write in pen (so that it wouldn't be permanent), and I would answer the phone or dial with some kind of alteration (wrong hand etc.). In an emergency, yes, all bets are off and ALL shabbat rules are suspended, but defining an emergency is tricky. To get into the med room, I wouldn't directly take my badge and badge in, but rather would lean against the wall and, oh look! the door opened as an indirect result of where I happened to be standing..! And that's the tip of the iceberg.
For me, it worked out that there was another RN who did regular Saturdays, and if I took regular Sundays it would mean a full weekend covered between us, which was amazing and NOT something I took for granted. I personally think you did the right thing in being up-front about your availability - though if it hadn't specifically come up in the interview I don't know that you're required to volunteer it.
I think, have a conversation with your Rav now before the next opportunity comes along, and see what advice they have. (Incidentally, I was once asked by the beit din (rabbinical court) I converted with whether it was okay for a nurse to work on shabbat and was told unequivocally that, overall yes, but there are many considerations!).
Wishing you much hatzlacha (luck) in your next opportunity, it should be smooth and a great fit for both your schedule and your professional development.
(Translations in parentheses for any curious readers!).