

Supported typing/facilitated communication is widely regarded as a pseudoscience. Studies have shown that FC is unable to produce answers not known by the facilitator. FC proponents believe that autistic individuals have the same linguistic ability as neurotypical individuals, and difficulty speaking is merely a motor issue.
As someone with autism, I can tell you: my brain can barely keep up with conversation. It’s not a motor issue. I have to actively think about appropriate word choice, how to structure sentences correctly, and neurotypicals don’t. If I don’t take enough time to finish the sentence in my head, the intonation is wrong, I’ll skip words, put them out of order, and just generally be unintelligible.
FC, like many other ‘theories’ surrounding autism, are made by people who have put years into researching autism, but have never thought to ask an autistic person anything about their experience.
Source: https://en.m.wikipedia.org/wiki/Facilitated_communication
I’m not sure I understand your feelings, but I’m going to offer uninformed advice anyway.
First of all, what does an ‘alternative option’ mean? Does the rest of your family pick a single thing off the menu, and you’re embarrassed you don’t want to eat the same thing, or is it more like you go by the drive through of another restaurant than everybody else? Picking different options off the same menu is generally the norm where I am, so I don’t think other people would find it that weird. If it’s the latter, I think most people would interpret that as you making a strong effort to engage and be supportive of your family, even it’s difficult for you.
As for your family being concerned about the amount you eat, there are a couple ways you could approach it. The easiest way would probably be lying about having eaten before. People are very unlikely to be concerned about you eating too little if you say you already ate beforehand. It can be a little bit rude if you know someone will be cooking in advance, though.
The second option would be saying you have a slow metabolism. This option wouldn’t completely stop your family worrying about your food intake, but over a long period of time your family will probably pick up the hint.
The third option would be to increase your metabolism through exercise, so you’re more hungry and eat more. This is kind of a weird option, but it also gets close to the root of the problem.
Regardless of which option you take, it seems like your family is trying to accommodate you, even if they’re doing it poorly. In these situations, being direct and honest can be very useful, since they are likely to accept your feedback. First of all, try to examine all the support they are already giving. If there are any situations when they anticipate your needs accurately, tell them that those situations are very helpful for you. If there are any situations where that isn’t the case, try and tell them why it went wrong and if you actually want support in that case. A very useful phrase is “I need to learn how to do X on my own.” It both explains why you want them to stop, while at the same time it doesn’t imply they’ve done anything wrong. Lastly, regarding the restaurant thing, try to be clear about your feelings, why you are embarrassed, and if you want help trying to solve that issue. They will probably try to brainstorm different ways to ease your embarrassment, and they might have different ideas than you.
If your family is being earnest about trying to help, the best thing you can be is earnest about the help you need.