Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Bangkok Intl to St. Denis as you normally would.
Yes, there are multiple flights from Bangkok Intl to St. Denis for under ฿ 56,904. The cheapest flight booked recently is on Multiple Airlines for just ฿ 55,916, but on average you can expect to pay ฿ 59,420.
Due to the rapidly changing circumstances due to COVID-19, airlines like Air Austral flying from Bangkok Suvarnabhumi to Saint-Denis Gillot have implemented new flexible change policies.
There are no direct flights from Bangkok Intl to St. Denis, but Cheapflights can provide you with all the information you need to find the fastest route with the fewest stops.
When travelling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Bangkok Intl is roughly less than St. Denis. You can assume this to be the case over the next 3 days.
Expect the weather to be around 36 °C on average in April. As the temperature peaks, try to get your plans done early, as flight price and availability will have an impact on your travels. The climate in St. Denis is generally colder than in Bangkok Intl.