Few things to adhere that i think will help us save the review to stay
1) Use email id that will look like a human not stuffed with words like mkt(marketing) or company name
2) Make sure email user name is properly set with customer name that looks like a person or individual is using and not a company.
3) Use different IPs to post the review. Use VPN and use same country IP for which customer you are passing the review.
4) Before you use any gmail id for posting, 10-15 days ahead search that company brand name, look for that company Google maps direction, open that website address, search for related service keywords being logged in that gmail id, so that it will imitate as if you are/were trying to find a product/service from that area and Google search history will be stored in your gmail web history and once that stored you can pass a review thereafter.
5) Before passing the review - pass reviews to restrurants, hotels , tourist atractions that are not commercial in nature, to show google as if you are staying in that area and exploring the city and adding the reviews and you are not only behind commercial business reviews that looks suspicious to flag.
6) Reduce frequency of review posting one a week or hardly 3-4 per month.
7) You can send your email list to your customer to send you invite to post a review with a link to their business page from their webmail.
8) Also make sure once review is posted, there is a return response to your review with a Thank you message for buying the product or taking the services. Responded reviews are hardly get deleted. Alert customer to respond the reviews or you login to GMB backend and respond them.