It would be very nice if I could tweak the default config on a RB750 to build a bridge on the LAN side instead the WAN side, leaving just one LAN port connected to a bigger, faster switch.

One port (the real WAN port) on a 4port switch goes to a modem, the other 3 ports go to other switches or devices I want to queue differently on the WAN traffic.

Is this possible? If so, how is it done? My first attempt clearly shows I have not read the documentation closely enough.

You can adjust the config as much as you want. Many of us do not use those default configs at all so it might be easier if you show your current config and describe what you want to achieve.