The BB middle line is a Moving Average, so for the: BB - 20, 2, Exp it is: MA(20) Exp. Close

For: BB crosses above MA 50
We can use: MA Oscillator - it is equal of the difference of two MAs
When MA1 = MA2 than the MA Oscillator = 0
BB crosses above MA 50 means: MA Osc. crosses the zero line upward

The strategy will be something like:

[Opening point of the position]
Bar Opening
     Enter the market at the beginning of the bar
     Base price  -  Open

[Opening logic condition]
MA Oscillator
     The MA Oscillator crosses the Level line upward
     Smoothing method  -  Exponential
     Base price  -  Close
     Fast MA period  -  20
     Slow MA period  -  50
     Level  -  0
     Use previous bar value  -  Yes

[Opening logic condition]
RSI
     The RSI is higher than the Level line
     Smoothing method  -  Simple
     Base price  -  Close
     Smoothing period  -  14
     Level  -  50
     Use previous bar value  -  Yes

[Closing point of the position]
Bar Closing
     Exit the market at the end of the bar
     Base price  -  Close

[Closing logic condition]
MA Oscillator
     The MA Oscillator crosses the Level line upward
     Smoothing method  -  Exponential
     Base price  -  Close
     Fast MA period  -  10
     Slow MA period  -  20
     Level  -  0
     Use previous bar value  -  No

[Closing logic condition]
RSI
     The RSI is lower than the Level line
     Smoothing method  -  Simple
     Base price  -  Close
     Smoothing period  -  14
     Level  -  50
     Use previous bar value  -  No

To be clearer I will explain further:

We will open at the beginning of the bar - [Opening point of the position],
but only when the [Opening logic conditions] are fulfilled
MA oscillator (20, 50) crosses the zero line upwards (Level line = 0)
and RSI > 50

We will close at the end of the bar only if either
MA oscillator (10, 20) crosses the zero line upwards or
RSI drops below 50.

Here we are using the BB middle line only, so we do not need BB, and the MA oscillator is enough for the

comparison.