It appears you're confusing fans in parallel with fans in series. This is parallel operation so, unless you're planning to also increase RPMs enough to overcompensate for the NF-A12x25s' smaller impeller (refer to the fan affinity "laws"), it's an increase in airflow rather than in pressure. For the Defines, P-Q charting suggests something like 90-95% the noise-normalized airflow of an NF-A14 from an NF-A12x25. As @ciarlatano just pointed out, most of the extra airflow interacts with the shroud and is unlikely to contribute much to upper motherboard cooling. So switching the front from 140s to 120s might actually somewhat reduce noise-normalized airflow over the upper motherboard.does this make sense?
With the Defines the most effective options for increasing airflow and lowering case temperatures are opening the door and pulling the dust filter. If you're not already doing that or just don't want to run the case that way then, given budget for six additional NF-A12x25s, it looks to me like putting the money into a Torrent, Lancool II Mesh, or even a Meshify is likely to have a greater effect on temperatures. See Gamers Nexus' thermal measurements in their reviews.
From the OP it's unclear to me why the CPU cooler and NF-A8 aren't adequately cooling the RAM. Changing out the NF-A8 may make a difference but I would give priority to understanding what's happening with cooler intake and GPU exhaust and checking whether the NF-A8 is promoting recirculation of CPU exhaust.
A bottom intake might help some with GPU temperatures, depending on what all is under the shroud, but my experience suggests slight, if any, upper motherboard effect.
For NF-A14 droning, the 140 mm fans for 2022 thread is currently right next to this one in the forum list.