I never understood the DCEU and DCU plans. Well, I do, they were stupid, but why they didn't flat-out copy the original phases of the MCU.
DC, more than Marvel, is made up of "solo" heroes with their own very solidly-defined corners of the DC Universe, and should be even more geared to the original MCU plan of the A-list heroes getting solo trilogies each and each numbered film leading into an equivalent-numbered team-up movie, versus how the Avengers' A-listers always had their corners meld into one another with the Avengers being so prominent a part of their stories/lives. Once you get the initial JL A-listers' first movies (Supes, Bats, Wondy, Flash, Lantern) out of the way, go into Justice League 1 with like Starro or so and introduce Aquaman (finding alien starfish in Atlantis is a natural introduction) and Martian Manhunter (comes to Earth to warn of Starro, I'd say), then keep up each trilogy, JL film, and "universe building" films (your Suicide Squads, World's Finest or Flash/GL team-ups, Teen Titans to spin Robin off from the Batman films, etc.) up to a Crisis on Infinite Earth equivalent. Each A-lister's solo film can see their stories advance between fighting major solo villains of theirs (Metallos, Riddlers, Circes, Rogues) and personal plot lines (Clark woos Lois and eventually marries her, Bruce adopts and trains Robin and they become the Dynamic Duo in his films, Diana Prince goes from military intel to curating a Greek museum in Gateway while wooing Steve, etc.).
Then once you do that.... either reboot naturally and start anew, or scale back and go back to some "smaller" adventures and build up again to a second Final Crisis trilogy or so. Focus on the new Young Justice generation as you give more prominence to adult Titans and keep the JL heroes' lives moving (Superman has Jon with Lois, Barry and Iris wed, Green Lantern deals with the Guardians' corruption or shitty back-up plans like in Johns' arc, etc.). But yeah, keep that reboot in mind so the universe doesn't get too bloated or become an aimless mess like the MCU post-Infinity Saga.