A pleasure to help! I've been using a Seasonic Platinum Fanless for over 5 years now, no problems so far.

I choose the PSU first to minimise noise. Thanks for the Sharkoon, which is an alternative I have overlooked thus far!

I would just pick the best case for my needs and then the best PSU for that case, not the other way around. My power requirements (it applies to the systems I build for others as well) are rather modest, so I just look for at least a Gold, but better Platinum-rated PSU with long warranty. It's usually an easy choice, having to pick one of a few Seasonic models. BeQuiet PSUs are also great, I built a system using one a few years ago, worked like a charm ever since.
Speaking of cases that can accommodate long PSUs, here's one with 24 cm: https://en.sharkoon.com/product/CA300H#specs 

Quote from: Codrut Nistor on April 09, 2023, 17:34:42I usually go with [...] as high-end as possible PSUs 

This would also be my choice. I have noticed that such PSUs start at 20cm length. Many cases have PSU allowances below or sometimes exactly at 20cm. Exactly 20cm is too risky, not to mention the cables. This leaves large cases. What cases would you recommend for top PSUs?

I have consider Phanteks P500A, Fractal Meshify S2, 2 or 2 XL, CoolerMaster H500 etc. I am sceptical about BeQuiet because their cases are half-closed at the front. However, these cases are huge. I would prefer something medium preferably not black, with Mesh, but whenever I try to give other manufacturers a chance, the PSU allowance is always the killer condition. I do not want a mediocre PSU.

I think that it can benefit being able to build your system using previous-generation parts in some scenarios. Depends on how often you replace your motherboard/CPU and what computing needs do you really have. There are still plenty of pre-2015 builds around, some of them still capable of reasonable gaming performance as well. Regarding brand-new releases, I agree with you, but depends on what market area you target. I usually go with low-end and mid-range CPU+motherboard combos, and as high-end as possible PSUs (depending on the budget, sometimes I build PCs for friends and relatives).   

It really doesn't even benefit the consumer anymore to be a DIYer barebones builder today.

Just for a depreciated in value mobo/CPU just in a few weeks (cooler, etc. not included), could tax a user $1200+ at release!

Think about that.