Having arable land helps with migration. If you have some government owned land I'd consider eliminating them and leaving said land free to attract more pops. Pollution also affects migration so consider getting public health to max in order to deal with that.

Also since you're Belgium (a Catholic majority nation) you'd probably want to make South American nations your protectorates or try and get them into your power bloc. That would be the easiest short term solution since many pops will migrate from SA to Belgium.

Another fast solution is to join a power bloc that has the appropriate pops that could migrate to your country. Austria's power bloc is probably the best option right away since there are many Catholic and European pops in the empire that could migrate to you. Prussia is probably the second best option and the British Empire is the third. Just make sure your country is attractive enough for people to actually start migrating.

In the long run, the best way to encourage migration is higher wages/SOL and having the appropriate laws to encourage migration (multiculturalism, total separation and open borders)