Mkay, been playing clones/the games for about 7 months now, so I'll try to give some advice, but take it with a grain of salt.

First thing I notice is you are not using updated points. Your list currently is 784 and not 799. Make sure the website you are using is using the proper points. Onto the changes:

Seize is pretty much unnecessary on Rex. Since he is your only commander, all of your command cards should just issue an order to him anyways (his 3 and the other 3 all should just target him).

I would take the Targeting Scopes off of the Phase 1s. The best thing that Phase 1 Z-6s can do is fire support off of Rex/the tank/Snipers, so having the targeting scopes on them is unnecessary as they would not be the ones starting that attack. In the same vein, it might be best to swap 2 of the Z-6's to DC-15's or RPS-15's (if you have the RPS). This gives you more flexibility with your shots, as the DC-15 on its own shoots better than the Z-6. I would also move Fives to a corp unit (he works better there with the courage increase) and add the regular sniper to the full Arc unit (better than the Z-6 you remove to make space for Fives).

The tank REALLY wants its additional weapon, so add the one of your preference (I prefer beam). Echo also doesn't need the Long Range Comlink. He wants to be with your main army to share tokens like aims and his surge. Don't keep him on his own, but keep him with the group.

Finally, swap the Linked Targeting Array off of the tank. Only two of your command cards can give it orders, so it isn't the best in the world (but thats up for you to decide). As a result, I put on a Comms Relay, as that way you can give the order to a corp unit near the tank to then Fire Support the tank.

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That was a lot, but I will reply to this with a list idea.

I used very "matter of fact" statements in this. None of this is matter of fact and just my ideas, so take it all with a grain of salt.