Adjustment in joint models

Hope these papers help, as they explain the rationale for adjusting certain variables in joint modelling:Joseph G. Ibrahim et al.

Basic Concepts and Methods for Joint Models of Longitudinal and Survival Data. J Clin Oncol 28 , 2796-2801(2010).

DOI:10.1200/JCO.2009.25.0654

And

vanOudenhoven FM, Swinkels SHN, Ibrahim JG, Rizopoulos D. A marginal estimate for the overall treatment effect on a survival outcome within the joint modeling framework. Statistics in Medicine. 2020;39:4120–4132. https://doi.org/10.1002/sim.8713

You could try several models with different combinations of factors in each submodel, before deciding on a final joint model.