Sounds like more of a landscaping forum question than a lawn care forum question, since there is no longer a lawn to care for.

...thanks - I already hit it again in mid-May with 16lbs. “Scotts Turf Builder Lawn Food - Starter Food for New Grass Plus Weed Preventer” (NPK 21-22-4, 0.5% Fe, 0.08% Mesotrione (Tenacity)). Works out to ~0.7lbs./1000sqft Nitrogen. I also leveled the old garden area with sand - it is still settling, but appears to be helping. I must say that the monthly 2x Tenacity and 1x Tupersan treatments have worked very well - almost zero broadleaf weeds or crabgrass germination to this point. Now comes the true challenge to keep it properly irrigated during the summer heat with no sprinkler system. I will plan to do 1lbN/ksqft in Sep+Nov as advised and keep updating this thread.

Yes it does look very nice from the street. And I can see where you would get picky about the quality. Before getting too deep into this, where do you live? With metric temperatures I'm assuming somewhere in the hottest parts of Canada or somewhere else in the world.

+1 forsheems. As he sort of said, you sort of screwed yourself by mowing first and then spraying the RU. You can recover and really need to slow down. Start now watering 3x per day to sprout all the weed seeds you can. Water every day for a week. That will help bring back the grasses you scalped off and did not die the first time. Then after a week, spray with RU again. Ideally you would continue watering daily to spray RU again in 2 weeks, but you're already off this program. The idea is to kill everything including all the potential weed seeds sitting on your soil. Then you could run the slit seeder without seed to pick up all the dead stuff. Set the slits to scar the ground about 1/8-inch deep. As you run over it you will find low spots that the slitter doesn't hit. You will also find high spots. I suggest running over the high spots over and over to loosen that soil. Then use a push broom to move the high spot soil into the low spots. Try not to get carried away with that process and create new low spots. Walk on the low spot soil to press the fluffy new soil down onto them. This is the time to perfect your soil surface. The smoother you get it now the fewer problems you will have with drainage. Tilling with a hand held tiller makes it almost impossible to get a good surface at the top. Tillers hop up out of the ground when they hit a root, rock, or hard spot. They dig deep when they hit sand or other soft spots. There's no way you can fix that. The result is the underlying surface is uneven. When you level the top of the soil that leaves differing depths of fluffy soil on top of the untilled profile underneath. When the fluffy soil finally settles in 3 years, it will settle to match the profile of the uneven surface below. That settling starts immediately and ends in 3 years, so your soil starts out getting bumpy and ends up really bumpy. That's why you don't till. If you had a tiller on the back of a 10,000 pound tractor, you can control that, but hand held tillers are the worst. Getting back to the seeding, once you have run the slit seeder over the soil without seed, you could scatter the seed by hand into the scarred up soil and get about the same results. Up to you. Either way roll the seed down to push it against the soil. If the area is small enough, just walk on it to press it down.