recs for a mesh system with TC capability for an old concrete walled metropolitan apartment in LA with a bunch of neighbors and outside wireless noise, that I can still get Time Capsule capability to work on?
First of all.. that is a difficult place to get any sort of wifi, even mesh to work in.
Second no mesh system has Time Machine capability. So that is going to need to be add on.
Is there any ethernet in the apartment? Is it possible to add?
I will assume the answer is no to both those questions.. but let me recommend you do think long and hard about running some ethernet. Depending on size and layout, it can be exceedingly difficult to get good speed through those solid walls. Even if it is just from one end to the other. What other cabling might already exist? e.g. phone or TV cable.
I need to ask what sort of internet / broadband you have?
Do you have a modem router supplied by ISP? If so make and model number.
What speed are you getting from it??
A relatively slow speed on your internet and no need for fast local lan .. exceeding 100Mbps say makes life easier. The faster you need things to run the more you need to spend.
I'd like a mesh network system, with time capsule capability networked like it is in my Apple ecosystem. This is to allow all the idevices, MBPs, iMacs, etc to backup.
idevices do not backup to a Time Capsule. Apple designed them to use either iCloud or local Mac. You can deliberately copy files to some local network storage.. but that is different to a backup.
The easiest solution to the backup problem of your Macs is to use a local hard disk. This is easy for iMac... simply plug a USB into the port at the back. For latest OS it is far superior method anyway. A laptop is more difficult as it makes backup dependent on remembering to plug in the hard disk. Let me therefore suggest a solution like ordering a charging hub with extra USB ports.. so every time you plug the laptop into charge it is then automatically connected to external USB drive and can backup.
Once you have this setup then idevices can backup to Mac in local network which will subsequently backup to local disk. Thus and so did Apple build it.
To do network backup there are two solutions..
Plug a USB drive into a main router which is NOT part of the mesh.
A good example is Synology router, RT2600AC.
The Orbi is still one of the best mesh solutions.. but it does not require taking the router role.. be careful as some mesh system MUST have main router as part of the mesh.
OR plug a network capable backup device.. i.e. your existing Time Capsule into the main router by ethernet even if it is part of the mesh.. this is meantime solution since Time Capsule will not last forever and Apple don't make them no more.. you need to get alternative and so far the best is Synology NAS. These can be small one or two bay units.. bottom of the range is fine.
Here is your tossup. A proper NAS has much better speed and functionality. The router as per my suggestion of the Synology is cheaper all up but is already getting a few years old and Synology have given no indication of AX wifi products. It could be a dead duck.
In terms of configuration of the router or the NAS to handle backup they are equal.. since the firmware in both is pretty much the same anyway.
Note .. I use Synology NAS.. it is not at all difficult to setup if you follow the tutorial. Lots of people are using them and find it is better than Apple Time Capsule for reliability with Catalina in particular since it uses SMB3. Apple never updated the TC to the latest network protocols. I would not expect them to do so now since it would require a major rewrite of the OS. In general routers are unsuited to Time Machine since it now requires SMB3 (AFP works but poor reliability). SMB3 is very resource demanding and requires dedicated devices like NAS which can be built with sufficient speed and memory to handle it. Routers for most part use the already deprecated SMB1 or AFP.