Earn a little bit of money by doing some simple jobs such as gardening or washing cars before turning to some low-stakes games to make a bit more money before turning pro.
You cannot install mods (including OptiFine) in Minecraft Bedrock, which is any version of the game that just says "Minecraft" in the menu screen, not "Minecraft Java Edition".
Playing poker isn't a sin. Gambling isn't good because it's not a very responsible way to spend money. If you just want to play to test your mental capacities and...
Unfortunately, no. As of the time this answer is written (Dec 1, 2020), the Bedrock Edition (formerly known as Pocket Edition) of Minecraft still doesn't allow for custom Superflat worlds....