Romania invests in education from the 11th of Nov 1918 making sure that all children get at least 7 years of schooling, that another 5 years is available for those who wish to do so, and that higher education is available for all of those who wish to do that. Invest in "night school" for adults where people can complete and or start subjects or courses they are missing. Make everything 100% free.
Invest in oil extraction and oil refinery make sure the state owns it all, and it is all run like a business.
Grant free healthcare for all.
Have free and open immigration for anyone around the world, as long as they learn the language AND that a job is waiting for them when they arrive, if there is no job that person is to be sent out again OR they can join the military or natural resource extraction companies, digging mines for iron, other metals, etc. And the person must of course do the actual job, if they do not then deportation, or if they find another job, then that is ok.
5 years to get citizenship.
Do land reform, where all larger unproductive lands that are not productive "enough" are given to the actual landless peasants working the land, if the land owners is just living of their labor and not reinvesting, then he is of no value to the state, he can keep his house and enough land to feed his family, the rest is to be divided up.
Pass laws that require a split between commercial banks and investment banks.
Give full and equal rights to all citizens, every person who is 18 or older has 1 vote.
Full and equal rights for women. They can do anything men can, such as having a bank account, having a job, owning a house etc etc.
Build factory to make modified versions of the 1911 with a bigger clip, 12-15 bullets, and something that protects against 1911 hammer bites.
Begin research and development of a submachine gun, test the versions hard, build a few 1000 of them and test those even harder, then when you have another version start mass producing that so that by 1931 you have 500,000.
Look at the Fedorov Avtomat, the Browing 1918 Automatic rifle, the French RSC M1917 and 1918 models, look at all possible semi automatic rifles and fully automatic, and try to build a Romanian version of a semi Auto, so that by 1931 there are 500,000 of them.
Build factories that make pieces of artillery that are at least 100mm and that have at least 4 wheels on them.
Build 3 factories for aircraft production, one for large bombers, one for some type of dive bomber and the last for fighters, do not actually start mass producing yet, just build small numbers of planes, for training and for development
Have the military train and study hard, but do not begin the material expansion until 1928.
From 1928 to 1931 build as much as possible of the most modern things you have, have the military do as much possible training using as much possible equipment.
When the greet depression hits in 1929, Romania should with the laws past be somewhat protected from it and even have an influx of people. Only take those for whom there is a need off, qualified people needed in different areas.
Then with that military and that country, start your invasions, first take the small countries, then the bigger ones.