Thanks for using the forums for such a great question. If you are looking to upgrade you phone you should definitely pick one up that is capable of housing two nano SIM cards at the same time. Some new phones that should fall within your budget and will support 5G are about to be released, the OnePlus Nord (two SIM slots) and the Google Pixle 4a (one physical one esim). Another good phone would be the Samsung Galaxy A71. Click here to see 2020’s top 12 dual sim smart phones. I was able to locate a neat cheat sheet for comparing Singapore SIM only providers and it looks like Zero 1 offers the best bang for your buck.
This information should ease your decision making process, let us know if you have any more questions.