How to improve my digital filter to extract DC from noises?
Your first problem is not insufficient design, but insufficient specification. Please note that 'as much as possible' is not a specification!

Rather than wondering whether any particular filter is sharp enough, and may introduce high frequency noises onto the results, it would be more constructive to specify how much attenuation is required at what frequencies, and then design a filter that gives this attenuation.

One of the first specification points is what the output sample rate should be. The lowest rate you mention is 10Hz, but then it is filtered after that. Is that the output rate, or does it come down? If your DC is varying at 1uHz, then even 10Hz could be considered to be rather over-sampled!

I'm not sure how you arrived at using a first stage filter of 320 taps? 

Anyhow, deep decimation like you're doing here will typically involve two stages.

First stage - get a large reduction in sample rate down to a few times your final rate, using a crude low pass filter, to have deep rejection nulls around your final sampling rate, nulls wide enough to accommodate your final bandwidth. This filter runs frequently, so will need to be short. This could be a CIC filter, though that requires very high precision, can need bignums for a large rate change. A low order decimating FIR is perhaps a better choice for a software implementation.

Second stage - design a final filter that eliminates the noise that has been aliased by the crude first stage filter, and drops the rate to the final desired one. This filter runs rarely, so can be very long.

Where the output rate does oversample your required bandwidth by a large factor, you can afford to be very inefficient with your filter design. In these cases, it looks like the case you describe, an iterated first order IIR filter is a reasonable choice.

The beauty of a first order IIR filter is that the effective length is adjusted merely by setting the value of k in the recurrence relation. You can either iterate it to improve the steepness slightly, or simply oversample a lot to allow the poor stopband not to influence your results. There is the very real problem that a small k and/or large decimation ratio can run you into numeric precision problems if you are not careful.

This may not be the easiest filter to design by forum. I am happy to help you firm up your spec and help you design it offline, just for the lols.