After my smartphone vendor's SMS app, and Google Messages both started serving me non-stop loans ads and gambling ads (never gambled ever in my life), I switched to Fossify Messages, and it has been going great for me. Never looked back.
They sometimes look like this, too [JPG]: https://static1.anpoimages.com/wordpress/wp-content/uploads/...
Deleting the Messages app for this is the same as deleting your email client in a bid to reduce spam.
There is a solution even without changing app: disable RCS. Everything real is fine with SMS, so ads are the only casualty. I wonder if this stupidity helps kill the already-struggling RCS.
Only time I need SMS, i.e. when a family member or a friend travels to a remote area, internet, and hence, RCS don't work anyway. So, I had to switch to good old SMS. Since I deleted all RCS stuff, I use Signal/WApp/Telegram 99.99% of the times, and good old SMS for the rest of the situations.