Simply omit the cucumber. Take some full fat greek yogurt, add some minced garlic, mint (optional but I love it with falafel), lemon juice, fresh dill, salt, pepper and olive oil. Mix well and let it sit for an hour or so to let the flavors meld.
Also, you might want to try a small tiny batch with the cukes, the trick to good tzatsiki is to grate the cucumber, salt it, and let it drain for an hour. Then you press all the water out of it with a tea towel. The flavor and texture is quite different from fresh cucumber.
I wouldn’t use celery, it’s too strong a flavor. Perhaps zucchini grated and prepped as above would give you the texture you are looking for.