You are really, really misrepresenting the usefulness of back aerial. Yes, you can SDI it. It's not a secret or a surprise. It's also not the end of the world, and if it wasn't obvious to you this was how it worked even when the game was pretty new, you just didn't have much foresight. It's also massively overrated how bad it is for Mr. Game & Watch. Do I expect to do 15 damage every time I land a turtle? Of course not. In fact, I expect only to do 1-2 hits. They are then knocked away from me and I can easily establish my optimal spacing to do... another turtle. As long as this keeps up, they are just forced to keep reacting to me. If they try to mix things up and get around it (which they will if they're smart), the zoning this ridiculously huge wall of hitboxes afforded me makes it pretty easy for me to deal with that as well.
You also seem to neglect one of the most important reasons turtle is really, really good and G&W's best move hands down. You can air control to a very non-negligible extent in the middle of the move. Here's how you shield pressure. Hit the shield with the first four hits, air control out of grab range (assuming they don't have a tether grab, yes if you had good spacing in the first place you can even avoid a grab from DDD), use the landing hitbox or the fifth aerial hit as a zoning tool if they decide to drop shield and run after you (mix it up). This does a lot of shield damage. If they start getting used to this, you can always mix it up and just let the fifth hit hit their shield which is blatantly unsafe, but it's fast enough that they can't punish it on reaction, only on prediction. That's actually a key point I'll come back to.
Utilt is his second worst move (his worst being Judgment Hammer). It's one of his few bad moves. Chef has all sorts of obscure uses though it's probably his third worst move so again you're picking from his worst here (note: try throwing out just one piece of food randomly; if you don't get hit for doing it, it was probably worth it). If you get a fully charged Oil Panic against some characters it kill at 0%. Do remember it's massively disjointed though at close range hits on frame 2. Just punish with it; it's not hard. Most G&W players are super antsy with Oil Panic and use it really soon or at really predictable times because it's so powerful and they only get one shot. It's much better than it seems if you base it off that performance, and even when uncharged your down special is working for you via bucket braking and protecting you from certain kinds of projectile spam. Before someone says"oh it's so slow they'll punish!" think about real matches where lots of situations happen, not the nonsense where they can teleport right next to you and hit you for ever doing anything laggy. I'm on one edge of the stage. The opponent is on the other edge of the stage and is Falco. Against most characters, his best option is to fire a laser. It probably won't actually hit, but who cares? It's free pressure with no possible downside. Even if there's only a .01% chance of hitting I'd use laser as Falco in that situation. Against G&W it isn't a good idea just because I could bucket it.
G&W's smashes are not that hard to land at all. They are extremely disjointed, dsmash and usmash have very low cooldown and charge release, and fsmash has massive lingering hitboxes. Sure they have fairly poor start up. It's fine; there are a lot of slow smashes that are not that unreasonable to hit with (Since when does Wolf have low smash speed? Is Meta Knight your average point?). You just have to... be unpredictable. The thing most people don't understand is that human reaction time, even among top level players, is not good at all. You can barely react to Ike's fsmash (even there I'm not sure if you can, though I know you can react to Falcon Punch and similarly slow moves); you reaction is honestly that bad. You don't THINK it's that bad because you use prediction to supplement your reaction abilities. However, if your opponent behaves in unexpected ways, you suddenly will get hit by "slow moves you shouldn't be hit by"... including all of G&W's smashes that have a bunch of really desirable properties that make them easy to use like this. For instance, what if I barely charge my usmash instead of just throwing out a max speed one? My opponent moves in to punish my usmash... and it goes off 2-3 frames later. There's no way to react to that; it looks like they just jumped into usmash. Everyone makes fun of them for being dumb, but in reality it was a very reasonable mistake for them to get hit. It's the same reason SDI in general does not ruin moves like the turtle even if you don't fully space it (which is completely safe even if not likely to do much damage). If they didn't expect to get hit by a turtle, they aren't going to SDI out of it. They'll complain that they are playing poorly, suspecting their own poor SDI as the cause, but in reality it is you who is causing them to fail.
The summary here is that predictability is a personal problem, and if you aren't predictable, everything is a lot easier to land. Constantly mix up very subtle timings and spacings, and don't do the same thing in the same situation every time. It's "hard" to do, but it's to be expected that the easy stuff and sloppy spacing that worked so well a few weeks into the metagame wasn't going to be eternal...