I hope that I am not stearing you in the wrong direction but here is a site that sells the hobby line of motors for the reloads.
https://www.a2zhobbies.com/Aerotech/RMS_Motor_Reloads
every thing listed as a 29mm reload is for the 29/40-120 case, from E-G.
when loading a E reload in the 29/40-120 case there will be a space with out any grain in it because it is a single grain eliment and when using a G reload it is a 2 grain unit so the case is full. These reloads are able to be shiped with out hasmat and no LEUP. As far as the 38/120 case there is a few reloads for it the G67 is a good motor and then you have the G339 which is a warp 9 motor that needs another sorce for the ejection charge because it does not have one.
Myself I am getting into the hobbyline of reloads by getting 4 of the 29/40-120 cases and clustering them together and also using them singualy also.
I have a few rockets that are built to take the 29mm motors. I also have 4 rockets that are built for the 38mm ones also thoses are mostly 3" dia and one 4" dia rocket, and one 2.1" dia rocket.
I did my L1 on a 3" dia rocket using the Cesroni pro38 2 grain on a H motor, I have the 1 grain case and a 5 grain case for the J motor and I am going to put a adapter tube in it to be able to use a I motor also which is a 4 grain motor.
I did my L2 with a AT J350 in the 38/750 case and also got the 120 case for the one grain motors as well so I can use it in my 2.1 amd 3" rockets.
The AT- 40, through 120 indivual cases will only take the reload that is for that case where the hobby line 29/40-120 will take all the reloads that is made for that case.
As far as a motor that is good for all around use I would go with the 29/40-120 hobby line motor for MPR and in the 38 size well that is to ones prefreance on what you want to use. I dont think there is much diferance between Rouse tech and Dr. rocket just differnt color and price.
Just rember that larger reloads will require hasmat to ship and a LEUP to buy and store unless you have the convieance of haveing some one in the club that has a LEUP or a vender that frequents your launches which my club has neither :cry:
I hope this helps.