Water will always flow to the path of least resistance. Running too much baseboard off a monoflow tee does not work because the resistance through the loop is greater than the resistance through the main. If you have the money the best thing to do is to pull the main monoflow feed and return altogether and run individual pipes ( I like 5/8" heat pex ) to each convector or radiator and then bring them back to manifolds at the boiler. A single circulator can be used with balancing valves or a single circ with thermostatic valves at the radiators. The baseboard loops get brought back to their own circulators and their own thermostats or they can be grouped as separate loops with theri own thermostat(s)