matlab - FIR-Decimation and Low-pass filter (taps vs number of input points vs number of decimation stages)

I am new to digital filters and trying to understand parameters behind them.

Scenario

I am trying to get DC value from signals sampled at 100kHz sampling rate. such high frequency is used due to system hardware limitations. A very sharp low-pass filter is to be used with cut-off frequency at 1Hz. The plan is to capture 10000 ponints first and then FIR-Decimate the income signal with several stages:

1. stage 1: decimation factor 10, FIR cutoff frequency 1/20 (128 taps), 100kHz->10kHz
   
   
   
   • income 10000 points, result 1000 points
   
   
   
   


2. stage 2: decimation factor 10, FIR cutoff frequency 1/20 (128 taps), 10kHz->1kHz
   
   
   
   
   • income 1000 points, result 100 points
   
   
   
   


3. stage 3: decimation factor 10, FIR cutoff frequency 1/20 (128 taps), 1kHz->100Hz
   
   
   
   • income 100 points, result 10 points
   
   
   
   


4. stage 4: decimation factor 10, FIR cutoff frequency 1/20 (128 taps), 100Hz->10Hz
   
   
   
   
   • income 10 points, result 1 point
   
   
   
   


5. Then a FIR with cut off frequency will be implemented with cutoff frequency 1/10.

Questions

1. In the FIR decimation, the FIR cut-off frequency is always stricter than the decimation factor to avoid aliasing, is this design necessary?


2. What are the tradeoffs of choosing number of decimation stages and decimation factors?



3. At stage 4, there are only 10 income points. However, FIR has 64 taps... Would the one point result still be valid?