I have spent a significant amount of time in the last few weeks making my AirPort network somewhat reliable and stable. I have had an uptime so far of 7 days on my ActionTec router and the network has been extremely stable. Even the wireless printers have been stable.
This is what I have:
1 - Telus ActionTec T3200 Router served by a Telus ONT with a 300/300 mbs up and down
1 - AirPort Time Capsule - latest gen
1 - Airpot Extreme - latest gen
2 - Time Capsules 1st gen. (one has the PS removed and soldered in wiring for external PS)
I have all but one 1st gen Time Capsule Creating wireless networks with channel selection in automatic and I have analyzed my spectrum and it appears the auto setting works quite well. I have no overlap.
What I have learned through lots of trial and error:
Moving into the future, I will look at upgrading all Airports to Ubiquiti APs and routers.... After all Airpots are at least 5 years old...... And I will connect an external USB to my MacMini for network Time Machine backup storage. I may consider a Synology NAS, but those are quite expensive and if macOS has designating a external hard dive for network backups, this may be a great way to go.
As for Time Machine, I think this may be phased out in the not so distant future as more and more stuff is being moved to the cloud. Seems as though this is the way that Apple is moving and everyone else for that matter.