but for whatever reason that's not super popular anymore
FoS are just plain better in terms of damage and board control for most of the armies. But if the meta is saturated with lumineth, I can see seraphon shifting back to DT.
New shootcasts concern me more, tbh. They can appear wherever they want and have high range.
As for lumineth - they are very dicy. If you can unbind speed of hysh, searing storm, protection of teclis and as much power of hysh as possible on their first turn, lumineth are basically helpless. It also depends on how good is your opponent at throwing 4+ to backfire spells at you. If stellar tempest/geminids go through, it will be devastating for them.
I'm also wondering how Coalesced Knight spam might work against them. Take a Firelance battalion and maybe some Terradons and try and hit them super hard turn 1.
I don't believe, that coalesced stand any chance against LRL. They depend on CP and Teclis has total eclipse. They have strong spells that play with bravery. They are good at character sniping and saurus lists need more of them.
If you are tailoring against lumineths, I'd go for builds which are good without hero support and have mobility. Kroak+Slann+Astrolith+guard combo is great. Salamanders, Chamelons and Kroxigors deal good damage. Kroxigors are especially interesting, because they have 4 wounds each, which means that riders won't have much attacks against them. What about dread saurian? It can tank lots of damage, deny their already poor movement, deal MW on charge, reduce already poor bravery. Use several chameleon units/salamanders to chip wounds from different units and then use Roar of Ruin and watch their army run away. Even razordons can see some play. Lumineth are bad at screening, don't have good saves and very high cost per wound. Razordons are cheaper and have easier time to reach cathallar/archers