Stacking in parts – less signal or the same (when all at once)?

Hi! I’m working on a long winter project: capturing the Horsehead Nebula + Flame Nebula using a 70mm refractor, smartphone camera (Galaxy S20 FE), no tracking, and short exposures (1–2 seconds).

Because I want to reach around 2 hours of total integration, I’ll probably end up with something like 3600 RAW frames.
But obviously my phone can’t hold all of that in one night, so I’ll need to split the project across multiple nights.

Here’s my question:

If I stack the data in parts (for example, 4 stacks of 900 frames), and then stack those 4 stacks together — will the final quality be worse than stacking all 3600 frames at once?

Some people told me that “stacking stacks” produces worse results, but others say it’s completely fine.
I want to make sure before I commit several nights to the project.

For clarity, this is what I plan to do:

My planned workflow:

• Night 1 → 900 RAW frames → Stack #1
• Night 2 → 900 RAW frames → Stack #2
• Night 3 → 900 RAW frames → Stack #3
• Night 4 → 900 RAW frames → Stack #4
• Then combine all 4 stacks into one final master stack

Stacking method:

I plan to use Average (or Average + Sigma Clipping) for:

• each of the 4 partial stacks
• and the final master stack

File format:

Each partial stack saved as 32-bit FITS (or 32-bit TIFF).

Is this the correct way to do it?

Will I lose any faint detail or SNR by stacking in multiple stages instead of stacking everything at once?

My setup:

• 70mm refractor
• Galaxy S20 FE shooting RAW (DNG)
• 1–2 second exposures
• Alt-az mount (no tracking)
• Darks/flats/bias applied
• Processed in Siril and PixInsight (sometimes BlurXTerminator)

Any advice or confirmation would be really appreciated!
I just want to avoid wasting clear nights if multi-stage stacking has downsides.

Thanks!