If you really want your hair to be straight, the only way to make it that way is to blowdry and/or flat iron it. Smoothing products aren't going to make your hair not curly (and as Seraphia said, will potentially just weigh it down with products you don't actually need).
One thing I will say about a lot of products meant to protect your hair from heat styling or making them smoother...a lot of them are more or less just silicones. If you are using sulfate free shampoo and like it, putting a bunch of silicones on your hair outside of the shower will ultimately dry it out. Sulfate free shampoo doesn't really do a good job removing silicones, and while they do smooth your hair in the short term by coating it, that coating keeps moisture out in the long run. If you like how your hair reacts to 'cone-free conditioner, I'd be wary of introducing any in your styling products.
This may not be an avenue you want to pursue, but I would personally suggest that you try an experiment (on a weekend, in case you hate it). A lot of the terms I'm about to start tossing around are highly google-able, so you might want to do that if you try this out. What you'll be doing is trying out the Curly Girl method of hair care, starting in the shower. Wash your hair as normal (since you're already using the sulfate- and 'cone-free products). Do the ACV rinse if you want to (I find it helps keep my hair smoother and easier to detangle, but it's probably not essential).
When you go to condition your hair, though, use whatever amount you normally do as long as it's enough to detangle. Then gently work out any tangles with your fingers or a wide toothed comb while the conditioner is in your hair. If you normally rinse out conditioner, do that. If you don't normally rinse it out, leave it in. However, whenever you're rinsing your hair, let it sort of rest in your cupped hands so the force of your shower spray doesn't make it straight. You want to sort of scrunch it and hold any natural wave or curl in while you rinse it. You also want to leave your hair pretty sopping wet when you finish your shower.
Next, find an old t-shirt (or microfiber towel, or anything made of knit jersey that you can wrap around your head) and use that to gently scrunch out excess water. I find it easiest to do this if I tip my head over (because it helps my hair start curling a little closer to the root). Once you've squeezed out enough water that your hair is no longer dripping, lay the t-shirt out flat on a surface that is about chair-height. Then, tip your head over so all your hair falls down from your scalp, and let it sort of pile onto the t-shirt until your head is touching it. Then you'll line up one edge with your forehead, flip the opposite edge over your head so it's against your neck, and twist the sides until you can tie or otherwise secure them behind your head. This is called "plopping" and you can probably find videos online that show the process a bit better.
Leave your hair like that for at least half an hour or so, to let it get a little bit more dry without the weight of gravity. You can leave it in longer than that if you want to (though I personally find that half an hour to an hour is really all that's necessary). When you unwrap your hair, do it with your head tipped over so you can gently extract the old t-shirt and shake out your hair. Then flip your head over and sort of shake your hair to let it settle.
The most important part of this process is the next step...don't touch your hair until it dries. Like really don't touch it. If you have a diffuser, you can speed up the process by using that on your hair, but you want to keep the setting as low and as cool as possible. I have a cup-shaped diffuser, and I let sections of my hair coil into the cup (while it's off) until it rests against my scalp, then I turn it on for up to 30 seconds per section, then turn it off before I go to do the next one. That gives my hair more volume and speeds up the air drying time, but you still don't want to touch it until it's dry. Also, don't comb or brush it at all once you're out of the shower (even with your fingers).
When your hair is dry, you can gently scrunch it up towards your scalp to make it look softer and give it more volume, but don't do that until it's completely dry (to avoid frizz).
Try that sometime and see if it makes your hair more uniformly curly. It might not, but it's easy enough to make it worth a shot. That particular method works best with a good gel or other styling product, but my favorite is DevaCurl (which is pricey), so you might not want to buy any until you've given it a shot without gel. I spent most of my life convinced that my hair was wavy-to-straight, only to discover that I actually have loose ringlets if I style my hair like I described above. I do have to do a lot to encourage my natural curl, but since most of it is in the shower, it doesn't take more time than just washing and conditioning my hair would.
I have also found that the texture of my hair has changed over time (even though I'm only 28). I had fine hair that curled when I was a toddler (and barely any of it), which turned into more or less straight thick hair as a child, which turned into frizzy wavy sometimes coarse hair as a teenager, which has turned into how-could-you-ever-think-this-was-straight less coarse hair as an adult. But I have also found, throughout that time, that I'm a lot happier with my hair when I work with its natural quirks than when I try to "fix" them. I actually flat-ironed my hair yesterday (and haven't washed it yet, so it's still straight today), and in addition to reminding me how much of a royal pain flat ironing my hair is, it's also reminded me that I really like it when my hair is curly. It's fun to be able to run my fingers through my hair all day, and I do like how it looks straight, but it wouldn't look this good if I was trying to wear it straight every day. My hair is healthy enough to tolerate that amount of heat styling occasionally, but I'd completely fry it (no matter what products I used) if I was doing it regularly.