Didn't want to make this too complicated although that's probably destined to failure
Anyhoo
This is my "How the Roman Empire could have been saved" short :
Making everyone a citizen seems laudable but it also exempted those citizens from taxation
Really the simplest thing would be universal conscription for Citizens alongside the option for non citizens to become so through military service. In this case anyone who isn't a former slave, a debtor or a runaway serf is classed as a Citizen.

The basic period of service is one year. The poorest sons enter a lottery at age 18 and are required to serve if/when they get called up. Everyone else has to serve four years and also pay scutage fees if they don't re-enlist by aged 30.

Noncitizens are free to join the auxiliary legions for a predetermined period after which they are entitled to join the regular Legions for four years in return for Citizenship

Correction: Serfs are able to nominate other "citizens" for the position of Tribunal (I talk about this more on my #UpScrolled account) but due to their unique position are totally ineligible for military service.
Obviously I'm not a fan of conscription but clearly the #Roman #spqr experience shows the folly of having a large standing army 🪖 which demands long periods of service