What Google's rules forbid
Two things. First, anything of value traded for a review: money, a discount, a free service call, a drawing for a prize. Second, anything that shapes who gets asked, which Google describes as discouraging negative reviews or picking out positive ones.
Both are in Google's content policy for reviews, and both get reviews removed when they're found. The first one people know about. The second one catches honest businesses that thought they were being careful.
The polite mistake
The survey that asks how did we do, sends the happy answers to your Google listing and routes the unhappy ones to a private form, is the exact thing the rule describes. It's sold as reputation management and it's a violation, however good the intentions were.
The fix is one line: send the same link to everyone. If a job went badly, you already know, and a call about it does more than keeping the customer away from your listing. A bad review you answered well is worth more to the next prospect than a bad review you dodged, and the dodge is the part that breaks a rule.
What to send
Get your review link from your Business Profile, under the option for getting more reviews. It's short, it's permanent, and it opens the review box directly instead of dropping somebody on a search page.
Ask right after the work is finished, while the driveway still looks new. Ask a question rather than for a favor: what job did we do, and how did it go. That gets a sentence about the work and the town, which is worth more than a bare star.
Email tends to get answered for this, and a text works when that's how you've been talking all along. Pick one, use it every time, and don't send four reminders.
What not to bother with
Don't write the review for them, and don't hand them a phrase to include. Somebody tested coaching keywords into reviews over months and found no improvement in the map, and a review that reads like it was dictated does damage with the person reading it.
Don't buy them either. Bought reviews get removed, they're easy to spot from the outside, and the profile they came from is a liability sitting on your listing.