Many options open; help me theorycraft please.

You're giving us only few options
For the mid-game setup, one beacon runner is plenty. I used to run 2x Galahad / Fujin / 2xBoa and it worked really well, very often netting me 10 gold a match. Galahads are fast enough to cap beacons if levelled up, and they can also defend the beacons. I guess my best "mid-game" setup of mediums would be 2x Plasmahad, Thunder Carnage, Orkan Rog, Thunder/Orkan Boa. (Never used a Rog myself, you could go for Taranosaurus Rog if you don't encounter too many shielded robots.) You could replace the Boa or Carnage with a Gary, but with the new matchmaking even a medium lineup needs some power in case your spawn point is attacked.

For the "end-game", once you arrive there we might have completely changed the mechanics of the game. Don't worry too much about it, your playing style and the robots you use will change over time. The main thing is also to have enough levelled weapons, as Leos, Griffins and to some degree Natashas are perfectly legit robots that can be bought for just a bit silver, so you can experiment with them and stick with what you like best. And it only takes a couple of days to level any robot to 7 or 8. You could jump to a heavy lineup immediately and experiment, chances are your "medium" lineup will face 80% heavies. You can level a bit asynchronous, that is, level one or two weapons of a kind higher than the other, because you never know which weapons you will use exactly in a few weeks or months. But with shields and anciles, both Tarans/Orkans and Mags/Aphids/Pinatas will be good weapons in the future, the question is only if you need 2, 4 or 6 of them. So no harm done in levelling 2, and switching them around.

Looking at your suggestion, it doesn't really fit your "speed/beacon capping" description, and many people prefer to have at most one of each robot type in the hangar, for more flexibility. Galahads will cap, but Lancelots are tanks, slow and heavy. Fury is slow as hell too, and I have regretted buying it as a Natasha does the job just as well.
I'd suggest to try one DB Griffin instead of a Lance, and a maybe a Carnage (Thunder/Trident/Zeus depending on your preference) or Rhino instead of the second Galahad, with a Galahad/Carnage (or Rhino)/Fury/DB Griff/Lance lineup. Try an Ancilot (Ancile Lancelot), a Thunder/Aphid or Thunder/Pinata Leo, or a RDB Griff, see if you like them.

You won your Fury?

It's incredibly slow, yes, but 3 heavies packs a lot of power. A lot of top tier brawlers hate Furies, especially if 2 or 3 are coordinated. Having said that, a good number of gold/top players don't run Furies, either. It's a matter of preference as well.

When playing solo, yes, the game forces you to play heavies, only exception probably is the Galahad. However, when you squad, bots like the Carnage, Fujin, Gareth and even Stalker can be given useful roles.