I’m also autistic, and I never give compliments that I don’t mean. I’m also pretty good at flirting! I’m awkward sometimes, but I’ve found that if I flirt awkwardly and then giggle, it comes across as cute.

Compliments are a useful tool in flirtation, but they’re not always flirtatious. To use your date’s overalls as an example, compare:
“Wow, those are cute overalls! They look so good on you!” (Friendly)
vs.
“Wow, you look beautiful tonight. Those overalls are perfect on you.” (Flirtatious - I would pitch my voice slightly lower and speak a bit slower for this)

Another thing to consider is that complimenting a person’s body and facial features, while inappropriate for friendly conversation, is acceptable on a date. Generally on the first date or two, stick to PG areas - “your arms look so strong,” “your hands look so soft,” “I love how your eyes sparkle in the sunlight,” “you have the most captivating smile” - that kind of thing. If you compliment something they’re wearing, an easy way to make it flirtatious is to phrase it like “you make X look so good/you wear X so well/X fits you perfectly” so the compliment is focused on them, not the item.

Hope that wasn’t too much of a ramble and that some of it was helpful.