Ok, wow. I mean WOW! You're getting the wrong advice on every front. St Aug would THRIVE in that shade. The grass died because they didn't water it. Actually it might have died the first time from a disease left to run out of control, but in any case, the solution now is new sod (NOT SEED) and water. That cheap builder grade bermuda is only one notch below the very best bermuda grass you can buy at any price. The reason it is cheap and "builder grade" is that it is easy to grow and every grower in Texas grows it. The supply of that stuff is inexhaustible, but you cannot get a better turf grass without paying through the nose. The name of it is TIF 419 or TifTurf, or something like that. It has been around for 40 years or more and is very popular. The more expensive TIF varieties are only used in professional sports stadiums. I should note that ALL seeded varieties of bermuda are basically common bermuda, not the TIF hybrid. Thus if you start with a TIF lawn, you should never reseed bermuda into it. The common bermuda looks like a weed in a hybrid turf. Here is a picture of common bermuda invading a TIF turf. Note the common is bluer in color, coarser, and has a different growth habit. It's a mess once you have them mingled. If you have already seeded with common bermuda, I would not attempt to install a hybrid (sod) lawn without a lot of prep to kill the common. June is the time to take on that project. Whatever you do, do not fall into the trap of watching all the golf TV where they are urging you to get out and do something to the lawn. Bermuda needs hot soil to do much of anything to it. The reason your neighborhood lawns might look cheap is because they don't know how to care for them. Bermuda needs a LOT of maintenance to keep it nice. It should be mowed to about 1 inch high, 2x per week and 3x per week during the spring flush of new growth. It also needs heavy fertilizer applied monthly all season long. Most people are unwilling to do what it takes to make bermuda look great. Having said all that, I still prefer a St Aug lawn if you can keep it watered. All lawns need exactly the same amount of water to keep them looking nice through the summer. That amount is one inch per week, applied all at one time. The problem with St Aug is if you fail to water it for 3-5 weeks, it can die on you. Other grasses will go dormant first, but St Aug just dies. That's what happened in your yard. Bermuda would never grow in your shade, but St Aug should do fine. In fact, St Aug will take over your yard even if you have bermuda now. You would have to keep the St Aug mowed at the mower's highest setting and allow it to invade the bermuda. Bermuda cannot invade into tall St Aug because the tall St Aug provides too much shade for bermuda. But if you mow the grass at the lowest setting, then the bermuda can invade just fine and will give you a mess. Absolutely DO NOT rototill the soil in prep for a lawn. You might want to rototill for a vegie garden but never for a lawn. The tilled soil will take 3 years to settle, and it will settle unevenly. There is hardly anything worse than mowing a bumpy lawn.

40 degrees or 40%? I don't know of any 4 stroke lawn mowers that are rated for slopes of 40 degrees. Oil starvation.

So sorry, don’t know why it didn’t upload with the post!

You probably would want to post in the Lawn Care (or whatever they call it) forum. And possibly in the regional forum that applies to NC, as both those would be more specifically related to your questions than would Landscape Design. You said you put down pre-emergent, but didn't mention applying any fertilizer. Lawns need it for greenup & growth, especially when irrigation systems are washing nitrogen out of the soil on an ongoing basis. Likely, you'll need pre-emerge 2x per year; there are winter weeds as well as summer ones. A post-emergent herbicide for centipede lawns (you'll need to check labels) will probably help you out with any broadleaved weeds. For grassy weeds you may need to dig them out (use a spading fork, not a shovel) and be sure to prevent their reoccurence in the future.