Apart from the plan that deducts from your CPF Medisave, it is advisable to take up the rider which can only be paid by cash.
Your NS Aviva Plan is a personal accident plan. Personal accident plans give a lump sum payout upon accidental loss of limbs, body parts etc. Disability income insurance is different from this - it pays the insured a regular income should he not be able to work due to accident or illness. For simplified illustrated example: John may insure himself for a benefit of $3,000 a month, and should he not be able to work due to accident or illness (medically backed up, of course), the insurer will pay him the benefit of $3,000 a month until he is able to work again.
IMO, the AVIVA SAF Accident plan and MyShield do not constitute comprehensive coverage. You should more adequately cover yourself with insurance first before moving on to investments. As a young working adult, your premiums are cheap, and priorities will be to insure the risk of premature inability to work.
Take this hypothetical scenario - let's say you only spend minimally on insurance and move on to investing $500 a month. Even if you manage to get 10% pa on your investments for the next 5 years, you will have accumulated about $40,000 when you are 27. If you are unfortunately struck with a premature inability to work at that age, is $40,000 sufficient to replace your income for the rest of your working years and prepare you for retirement? My advice will be to take care of your insurance needs first, then even think about investing. You have time for investments, but you cannot control if or when something might happen to you, especially during your early working years.