Cisco SG300 as core switch, Router needed for separated vlans

The configuratior allows you to sign into the web page, and then launches Java where you sign in a second time. After that it just stops working.

If you're running into java problems, it sounds like you're trying to use Cisco SDM to manage your Cisco 850. Cisco SDM is EOL, and you should use Cisco Configuration Professional instead of SDM. Cisco Configuration Professional supports the 850 series routers.

If I can get into the router config once, is there a way to get this router running with some firmware that doesn't depend on Java? I've flashed TP-Links with DD-WRT successfully, but DD-WRT doesn't seem to run on a Cisco 850.

You're in luck! You don't need Java to access a Cisco router. All you need is telnet or a serial cable... they commonly look like this:

Options:

• You could try to telnet to the router's IP address, although that depends on whether someone previously had telnet configured on the router.
• If you can telnet to the router, but don't know the password, then you'll have to perform password recovery; this is a handy mapping of break sequences for various serial terminal emulators. This method is also useful if you can't telnet to the router.

You have an elegant physical setup, but logically, it’s deficient in a lot of ways. You’re relying on layer 2 for all of your connections, which doesn’t provide any real failover your setup should provide. Luckily, you aren’t wasting any bandwidth with STP because you don’t have any redundant links; VLAN 100 and VLAN 200 only have a single way out, if I’m reading this correctly.

I can't implement multiple area OSPF because the CPE public subnets are too difficult to summarize and the public range is split over several POPs without hierarchy, so this would be inefficient.

I don’t even think you need to go as far as setting up multiple areas, the segment your concerned with is fairly small. Cisco recommends you stay under 50 routers per area, though, most will agree that you can exceed that amount by quite a significant margin and still be considered healthy. I have about 80 routers per area with no issues; Ron has even more than that without any hiccups.

• Implementation of EtherChannel between the access switches and CMTS.

This all hinges off what happens higher up in your network. If you’re significantly oversubscribing your distribution to core links, then you may not see any real benefit in increasing your bandwidth at the access layer. The same holds true if you’re doing this with your access to distribution.

Again, as most have mentioned in the comments, implementing OSPF would be pretty easy. If you were utilizing a layer 3 protocol, you would share that load across equal/unequal cost links, too. Perhaps a redundant link between your distribution switches would allow each access switch a redundant path out. As it stands, if one of your access links goes down, you lose an entire subnet (VLAN 100 or VLAN 200).