I think some of the ODE solver stuff in PyStan has been difficult for the maintainers to support (ODE in PyStan: 'Failed to parse Stan model')
You might try cmdstanpy: https://cmdstanpy.readthedocs.io/en/latest/index.html which is a different interface which might be easier.
As @emiruz says, the Stan part of ODE modeling should be the same across the Python/R interfaces (as long as you’re able to get PyStan with the appropriate ODE solver). Here’s a nice example of using ODEs in Rstan: https://mc-stan.org/users/documentation/case-studies/lotka-volterra-predator-prey.html
edit: Pystan might support the rk45 solver without these difficulties, I’m not sure, maybe @ahartikainen would know.