I do tend towards the long-winded - it is my second retirement hobby after music...
My perspective assumes a general-use PC, one of those uses being Hauptwerk. If your target PC is solely dedicated to HW, different optimizations might be called for.
Totally understand. I can say for the modern AMD CPU, chip insertion is extremely easy, using a ZIF (Zero-Insertion Force) socket: flip up the lever, drop the processor in (no force beyond gravity), flip the lever down to secure. Don't over-use your thermal paste, just a pea-sized dab. When I formally tested thermal solutions, I was repeatedly pulling my cooling tower, removing the CPU, cleaning off old paste, reinstalling the CPU, applying new thermal paste, reinstalling the cooler - <5 minutes per cycle, mostly spent cleaning off the old thermal paste.
I think there are a few reasons for home-building, none universal: some find it fun/educational; more control over parts, possibly saves some money, but often not as much as one would think. A pre-built will generally have a system-level warranty, also possible for a custom job. If you build, parts will be under warranty, but not your build. There are now many YouTube videos demonstrating build techniques - I found those helpful, as I hadn't home-built in 20 years until this last machine. Had my college age son do 90% of the work, it took us about 2 hours from parts sorted on the table to booting Windows - and that was a fairly complex build... If you are getting a custom job, you should be able to get the quality parts matching your needs, and the builder may be able to make recommendations - but be careful, they may just be touting what they have (think waiter's recommendations at a restaurant...). I would do my own research before committing to a builder's suggestion(s), but I would definitely listen.
For non-custom commercial builds, OEMs often save money on Power Supplies and MOBOs. PSUs will often be rated for just enough power to support the initial config (little headroom for expansion), and are often a bit on the cheap side. Over the years, most PCs in my family have been non-custom, off-the-shelf pre-builts purchased from big-name retailers: every eventual failure of those units had a PSU connection. One pro-tip on selecting a PSU: add up your total everything-running power draw, and double it to roughly determine your ideal PSU power-rating. PSUs draw power based on current need, not on their "rated max". Their designs tend to be most efficient in terms of least power/heat waste at around 40-60% of their rated max. So an over-provisioned PSU, all things equal, should draw less power, emit less heat, and run quieter. Marketing terms like "Bronze/Silver/Gold/Platinum" represent actual industry average-efficiency standards in increasing order. In my region, "Gold" is the sweet spot in price/performance. Many good PSUs can/will run under normal load without spinning their fans (quiet!), and will generally advertise that capability.
Last tip - if purchasing a Windows license yourself, be aware that many (most?) online sellers listing at a large discount from Microsoft's retail list price are at least a bit on the shady side. Many of those licenses are "real" (not hacked), but not necessarily what they advertise - they may be keyed to old activation codes for defunct versions of the OS, and you may find that while you can activate them successfully once, they may not be allowed to activate on a successor PC, or under a MOBO swap-out on the same PC. Some of them are quite intentionally fraudulent, using old keys (say Windows 8.x), but creating a proper modern MS-looking USB stick with Windows 10 media on it, and reusing some authentic MS packaging - designed to look authentic but totally bogus - mislabeled licenses, authentic stickers removed from original MS parts and re-stuck to knock-offs, and other such nonsense. I worked with MS on tracking a few of those down. In my experience, they activate at least once (i.e. MS will activate the first time using the supplied code), but after that, you may be on your own. I'm guessing not necessarily illegal for the most part, but not fully sanctioned by MS, and contractually in a gray zone, and certainly misleading. If it activates, MS will supply updates/patches (the license is good), but may not honor a future transfer to and reactivation on a new machine, or a replacement MOBO. Unlike OEM Windows, a retail MS OS license entitles you to redeploy the license on a series of machines, as long as it is one machine at a time. So you could buy once, and upgrade/change-out your single PC multiple times reusing that same license, as long as it is one machine installed and activated at a time. OEM Windows is bound to the single original physical machine, and cannot legally transfer to a later replacement PC - new PC, new license. For that restriction, MS charges the OEM less for the license. A custom builder may provide multiple options here, but be sure you understand what the license restrictions will be - single use or transferable.
Cheers, Bob - and good luck.