Natstrackalpha
1. In a GA, CLB mode (managed mode) is not available but only OPCLB.
2. SRS mode cannot have select speed. If approach was done in select speed it will automatically change to managed speed and in SRS if you change to select speed SRS will change to OPCLB.
3. THR RED and ACC altitude have different functions. Whenever you bring thrust levers to CLB, thrust changes to THRCLB and auto thrust to active ATHR from armed i.e. from blue. So now you have overspeed protection. SRS will continue till ACC ALT.
4. If you do not bring thrust levers to idle at THR RED altitude as long as you are climbing FMGS will restrict speed to VFE by increasing climb rate but it will no do so in ALT since ATHR is not active and you risk overspeed. Does it answer your question?