AirPort Time Capsule 802.11n (3rd Generation) w/ New 2019 iMac

Wow.. your list of networks within range is incredible.. no wonder you have issues with disconnections.

You need the very best most powerful AC wireless router you can get. And you want to be able to select DFS channels if at all possible. Your best bet is 5ghz end of the spectrum. 2.4ghz will be totally unusable.

I would buy Asus high end model or Synology RT2600AC as they have best interface and most Apple compatible with excellent quality wireless.

If you need to get wireless to extend to other areas of the apartment, and are getting dropouts then use ethernet or powerline adapters and use another wireless AP or use Mesh gear. eg Netgear Orbi.

When you are in areas of very high wireless saturation distance is everything. Plan to have wireless access point (WAP) in the room .. don't expect to get wireless even in the next room.

It would also pay you to wire as much as possible. It seems popular these days to have wireless for everything but devices grouped together can work a lot better wired.. e.g. you entertainment areas.. TV, media player, game console?, Blue-Ray player, Home Theatre Amp.. everything that has ethernet .. plug it in and turn off wireless.

Your old TC is not working too badly considering its age. But you are in environment where you will struggle with even the very best equipment.

The A1355 (Gen3 TC) is not very different from the Gen4 TC .. the big difference being a much better wireless card with superior wifi in the Gen4.. all the same the Gen3 with the better 2TB drive is identical in other respects to the later one and may prove to outlive them.

The wireless performance of the Gen3 was not great. And that is one area that does seem to suffer deterioration with age.

Particularly the higher band.. 5ghz.

Let me suggest you do a wireless survey .. Apple includes a wireless diagnostics.

Here is what my scan looks like.

If you click it you can see full size.

TCA87F47 is a Gen4 TC located about 2M from the computer.. and the unifi 5Ghz is a Gen5 TC located 1M from the computer.

Signal levels are dramatically different. -25dBm vs -41dBm

Channels also matter. The Gen5 is running a DFS channel 140 and the Gen4 TC is using 149.

You can hold down the option key while clicking the wireless fan to find the actual link speed.

It also shows if you hover your mouse pointer over the link.

450mbps is the best speed a Gen3 or Gen4 TC can manage on 5ghz.. and usually only very close up.

Signal and speed at the TC end is equally important.

The only setting really worth playing with in the airport utility wireless options is the channel for 5ghz.

Low channels are better at solid penetration but higher channels get superior amplification which often gives them far superior signal. I do suggest to try 36 and then 157 or 161 just as a comparison.

Putting real numbers will help see if the new iMac is actually simply getting poor signal or something else is wrong.

I would recommend replacing the TC with a wireless router.. high end Asus or Synology.. but lots to choose from.

I pick those ones.. particularly the synology as it is Time Machine compatible with USB3 hard disk.

You can then plug the TC into the new router with wireless off and set in bridge mode.. that is how my Time Capsules are configured.

Congrats on your remarkably good luck with the Time Capsule. Unfortunately, the internal power supply and hard drive should have failed a few years ago, so frankly the Time Capsule is living on borrowed time. It may go at any minute, or with great luck, it may go another year or two.

In other words, you really would not want to store any data on the Time Capsule that you cannot afford to lose, unless you have the data backed up on another drive.

I'm not clear on whether you are backing up the new iMac to the Time Capsule over the Time Capsule wireless network, or whether you have another WiFi router and have attached the Time Capsule to that router using a wired Ethernet cable connection.

A tiny "blip" of the Time Capsule power supply can cause the WiFi network to act up, and that may or may not be what is occurring. If you want to continue to back up over WiFi, it would be better to not use the Time Capsule wireless......if.....you have another WiFi router that you can use for the wireless service on your network.

the 3rd generation TC overall has functioned as expected and I have not encountered any major power or hard disk failures since I bought the product some years ago.

Just to be clear, the average useful life of this version of the Time Capsule was about 4-5 years.

I have experienced a few minor backup failures in which the sparse bundle files have become corrupted or damaged in some way, and have had to be re-created. I'm unsure if that's related to updates to the Mac OS or due to another problem

You may have a different opinion, but corrupted sparsebundles are not "minor" failures, since you lose all of your backups for a given Mac when this occurs. Unfortunately, this is a fairly common issue with the Mac and Time Capsule, and it is difficult to point to the real culprit for the issues. Some operating systems have more problems that others in this regard, but the issue has been with us for many years. Older Time Capsules are simply going to be more prone to experience backup problems.

I also backup each desktop Mac nightly using SuperDuper to create a mirrored, bootable hard drive to an external drive as well as using iCloud, OneDrive & DropBox for personal files and Box enterprise account for my employer-related files.

Good plan to have an additional backup strategy in place.

Again, backing up data is not an issue for me since backups are being processed by the TC on all computers. The issue I'm having is only with the networking capability - not with the hard drive backup.

And I am sure that you understand that when.....not if.....the TC hard disk fails, you will very likely lose all of the backups. Personally, I would not trust any hard drive with 8 years of use on the product, but again this is your call. It will really help that you have an additional backup plan in place.

The 2019 iMac frequently drops the TC wireless network (which interestingly seems to only occur with the 5 Ghz network, the 2.4 Ghz network remains connected).

LaPastenague has already responded on that issue and we should know more when you work through his suggestions. 5 GHz has always been a maverick of sorts. Faster speeds are attainable with 5 GHz, but it is not as stable as 2.4 GHz in my experience, since 5 GHz signals are generally weaker signals compared to 2.4 GHz.

The new iMac is capable of 802.11ac wireless, but your version of the Time Capsule cannot do 802.11ac. That may or may not be part of the problem trying to get products that were designed in different decades working well with each other.

In general, 5 GHz works best when the router and computer are in the same room or very close proximity to each other. I have found that if the 5 GHz signal must pass through only one wall, the speed drops by about half. Your experience may be different though.

To avoid confusion, I am not going to comment on the wireless troubleshooting that you will be doing with LaPastenague unless I have some significant and different input.