At high sensitivities like 1 mV/div, the dynamic range is determined solely by the frontend noise. Even a totally noise-free 50 ohms system (resistive noise only) cannot provide even 7 bits of dynamic at 1 GHz bandwidth. At 500 MHz it is still less than 8 bits. In practice, even the lowest noise frontends exhibit at least twice as much noise as in the previous calculation, thus reducing the dynamic by another bit. Therefore 14, 12, 10 or even 8 bits don’t make the slightest difference at 1 mV/div.
The FFT reduces the bandwidth, hence lowers the noise, so we can get more dynamic range for individual frequencies. But this is due to the process gain of the math operation and does not benefit from more ADC-bits either.
The real difference can only be seen at the lowest sensitivity without attenuator, like 100 mV/div on the Siglents. Once again, with a noise-free Frontend, we could get 13 bits of dynamic at best. With a real low noise frontend and 2 nV/√Hz the dynamic is limited to 12 bits at 1 GHz bandwidth.
Higher bit counts may be beneficial at lower bandwidths, down to 10 MHz at best, but not lower, because the noise floor rises significantly at lower frequencies. At 100 mv/div and 20MHz bandwidth, we could make use of 15 bits if we have a very low noise frontend with only 2 nV/√Hz at 20 MHz. But even at such low bandwidths, it is less than 9 bits at 1 mV/div.
Long story short: for 1 GHz bandwidth, 12 bits is the absolute maximum sensible resolution, fully exploitable only in three vertical gain settings: 100 mV, 1 and 10 V/div. More than 12 bits make sense only for low bandwidths and low sensitivities of 10 mV/div and above. Thus 14 bits is of very limited value on a general purpose DSO.
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