Ujm, there were and are people who are shomrei Torah who for whatever reason have made Shidduchim themselves. Many met in Brooklyn college in the 80s. That doesn’t mean it’s the standard of tznius in the Yeshiva world, or that bnei Torah would do it, but if one observes a woman who he is interested in marrying, it’s not assur to approach her, as long as it’s clear that he’s intended on marriage and nothing else. There needs to be a clear distinction between what’s assur gamur, what’s the proper decorum for a yid, and what is the standard of bnei torah. There’s a difference.
As to the OP, i doubt it’s her first time going out, so all you have to do is make a phone call saying what shadchonim usually say… You had a great time but you don’t feel the shiduch is for you, hatzlocha rabba and have a wonderful day. It doesn’t have to be with any details(that can be more hurtful). I don’t suggest texting. While she might not pry for more info on a phone call, on text people say things they normally wouldn’t say, and the confidence and assertiveness that one can employ on a phone call is missing in a text. I also don’t feel it’s tznius to text women in general.