We perform high-resolution spectroscopy of the 3d D23/2-3d D25/2 interval in all stable even isotopes of CaA+ (A=40, 42, 44, 46, and 48) with an accuracy of ∼20 Hz using direct frequency-comb Raman spectroscopy. Combining these data with isotope shift measurements of the 4s S21/2↔3d D25/2 transition, we carry out a King plot analysis with unprecedented sensitivity to coupling between electrons and neutrons by bosons beyond the standard model. Furthermore, we estimate the sensitivity to such bosons from equivalent spectroscopy in Ba+ and Yb+. Finally, the data yield isotope shifts of the 4s S21/2↔3d D23/2 transition at 10 parts per billion through combination with recent data of Knollmann, Patel, and Doret [Phys. Rev. A 100, 022514 (2019)PLRAAN2469-992610.1103/PhysRevA.100.022514].