clogged drain

I have a small pool with one inlet just below waterline, one outlet just below waterline, and one main drain. Only one pipe to and from the pump/filter area. My main drain sump has two holes, one with a plug in it. Don't know if this is for pressure relief or what. No valves. No skimmer niche...

We’ve had a ton of leaves mixed with ash from recent fires, and I think the drain/pipes are clogged. There’s no pressure in the filter, and the skimmer is barely sucking any water. Tried flooding the filter and skimmer basket with a garden hose, but it hasn’t helped. Any suggestions?