Retrofitting substantial new work into a roughly fifteen year old integrated design with (minimally) roughly seven year old hard disks, and with BCM4360 that can’t manage Wi-Fi 6, or Wi-Fi 6E, or the current Wi-Fi 7, and all this in a product segment that Apple exited in 2016, seems very much a substantial investment in the creation of support headaches and missed opportunities.
Those old hard disks are already failing, before any upgrades, too.
As for implementing and maintaining network storage protocols, Apple had many choices — including AFP, NFS, and SMB — and SMB is the easy choice there. More recently, AFP is gone, and NFS is fading.
The most Apple-like of the network vendors is probably Ubiquiti, with a massive selection of wired and Wi-Fi and other networking hardware available, and they’ve been just starting to offer network-attached storage. The first-generation dedicated UNAS Pro product includes Time Machine server support.
As for other available NAS options, Synology has a much wider selection of NAS boxes. Most (all?) of the Synology NAS boxes have time Machine server support.
Similar to exiting Wi-Fi, Apple also exited the server market at macOS 10.14, and Synology NAS and macOS itself can replace most of what the macOS server product had provided.
Most any NAS can allow local backups using Time Machine, or using add-on apps.
Some NAS products, including Synology, can then mirror those backups elsewhere. UNAS Pro has ”support for Dropbox, OneDrive, and Google Drive” and with “S3, BackBlaze, and Wasabi integrations launching soon”.
Where such both server and data redundancy is appropriate, I’m aware of one vendor offering 800 km spans for their distributed disaster-tolerant shared-write server and storage clustering products, but you’re not likely willing to pay those prices.
As for the “the customer is always right”, I’m skeptical. "If we adopt the policy of admitting whatever claims the customer makes to be proper, and if we always settle them at face value, we shall be subjected to inevitable losses." Put differently, some customers can be just too expensive to service, and most businesses have effectively “fired” certain customers, and that separation for a variety of reasons.
TL;DR: The market and the competitive product offerings have changed immensely since 1999, and the era of AirPort and AirPort Time Capsule is ending.