Indeed, with Thai ROP you will get 100% miles on any paid ticket. Of course not on the award tickets. If you will fly Singapore Airlines, you will only get the SQ mileage on your Thai account. On the discounted economy tickets this means no miles.
Actually Thai has a very good FFP for people who aren't flying very much or only in Economy. It's quite easy to get Silver (10.000 miles) and even Gold doesn't require so many miles (50.000) and it gives you all the advantages of Star Alliance Gold. And even then you keep your status for two years.
I also experienced no problems untill now with getting award tickets and using my annual upgrades, even for very long flights (e.g. Brussels - Perth in Australia).
On the other hand the Brussels Thai Airlines office is so small I know all the empoyees by name and they know me. We do fly frequently to Thailand since we have a second home over there. And bringing a box of Belgian chocolates when picking up the award or upgraded tickets maybe helps too (Thai style).
Henk