When a player changes their doctrine away from Fire & Movement, they stop building jumptransports on their capital. A lot of new players seem to get blindsided by this since they can't expand their empire without a continuous supply of new jumptransports. Changing doctrines also harms efficiency on the capital, which is not indicated in the interface and is an unnecessary burden on new players.
I would suggest that players not be given the option to change doctrine away from Fire & Movement unless their empire contains a dedicated jumpyard and a jumpship autofac.
Alternately, it might make sense for the imperial capital or all capitals to be always be able to build some form of transport and infantry under any doctrine other than T&E. Maybe S&H could build mech infantry and a new class of large-capacity gunship-speed transports with missile protection, in keeping with this doctrine's martial theme.
A third possibility would be to have a more informative prompt before switching doctrines that explains the consequences of a doctrine change. Doctrine changes are infrequent and significant so it makes sense for there to be some friction in the process. The prompt could explain how resources can be exchanged for jumptransports via Mesophon. New players would need to start with enough aes to buy a couple thousand jumptransports.