I understand with the DC coupled system, the inverter will stay on and produce solar during a grid outage.
You mentioned tigo optimizers, and operating without batteries.
If the grid were to go down while PV power is available, things would continue to work.
The problem is what happens when the grid fails during no PV production. This is known as the 'dark start' problem.
Without batteries, there is no power source to keep the tigo CCA alive. So when the sun comes back out the panels will still be shutdown because they aren't receiving the power signal for the CCA.
There are several things you can do to address the dark start issue. If you add batteries, eg4 has a document about using the CCA to deal with dark starts. But if thats not an option..
Have an alternate power source available to power the CCA. A 12v battery.. a small solar panel with a 12-20v output, a small UPS and a wall wart plugged into it, etc. It'd just require a manual process to start the 18kpv without grid power or batteries, when using RSD.
What I did was wire the power input on the CCA with a 30a anderson powerpole. Get a 12v wallwart and put a 30a powerpole on that. During normal operation, plug the wallwart into an outlet served by the inverter (so that when RSD is triggered the outlet loses power and shuts the CCA off, complying with RSD requirements). Then, I added an anderson powerpole to the batteries on my electric gate opener.. (if you don't have one, you can just buy a small 12v 5ah battery, and keep it trickle charged). If the power goes out and I'm in a dark start scenario, I just disconnect the powerpole from the wallwart connection, and plug the CCA into the battery to get the PV arrays going again.