Building some kind of high-end workstation (graphic design + ai + gaming a bit), having some noob questions

Building a pc for the first time, having couple of questions:

1) is applying thermal paste on a new cpu necessary?

2) why so much people strive for water cooling if air cooler can handle enough Wt?

3) pcpartpicker has a disclaimer: "some physical constraints are not checked, such as RAM clearance with CPU Coolers." Could someone tell what else constraints are not checked, what should i check manually?

4) if i have 2-rank 2x32 gb RAM and would like to upgrade to 128gb in the future, can i just add the same 2x32 planks? or i will need to sell it and get 1-rank 4x32 instead?

5) is 1200w PSU really an overkill for 4090? I saw in other posts that even 850 works for 4090, but also some people say that it is good when decent % of PSU limits is free.

parts list: https://pcpartpicker.com/list/LfjTbL

CPU: AMD Ryzen 9 7900X 4.7 GHz 12-Core Processor

CPU Cooler: be quiet! Dark Rock Pro 4 50.5 CFM CPU Cooler

Motherboard: Gigabyte B650 GAMING X AX V2 ATX AM5 Motherboard

Memory: Kingston FURY Beast 64 GB (2 x 32 GB) DDR5-6000 CL36 Memory

Storage: Kingston Fury Renegade 2 TB M.2-2280 PCIe 4.0 X4 NVME Solid State Drive

Video Card: Palit GameRock GeForce RTX 4090 24 GB Video Card

Case: Lian Li LANCOOL III RGB ATX Mid Tower Case

Power Supply: Deepcool PX1200G 1200 W 80+ Gold Certified Fully Modular ATX Power Supply

pc purpose: graphic design (adobe illustrator, photoshop, maybe some after effects too), working with ai (stable diffusion & FLUX: image generation and training Loras, maybe using some local LLMs), maybe some 3d too (Blender); gaming also included (something like rdr2?)