The Ascension process isn’t something that should be undertaken lightly, although it is expected that this will be the eventual outcome for any colony founded with a sense of permanency. You are right to make the ask - what’s the catch? What will be expected of us? What benefit will we get? I hope that my response to your query will leave you with a positive assessment of the benefits of Ascension.
The first step to Ascension, from my reading of how the Commonwealth ascended from a colonial endeavour financed from Luminaire to a self-governing member state in it’s own right, is ensuring that the colony is able to govern itself properly and effectively. This would go hand in hand with the requirement that it can sustain itself to a certain degree befitting that of a member state, rather than be wholly reliant on the metropole, to ensure that the standard of living for those living there is not prejudiced through avoidable factors.
The agreement to abide by the terms of the Federal Charter/Constitution and Federal law would also be a precondition - although ‘opt-out’ clauses do exist. The prospective member state, or a nation-state outside of the Federation seeking to join the Union, should review these articles as they would form the basis of the contractual arrangement between them and the Federation.
Politically - you are correct, voting rights/representation. However, this should already be the case as it is the sponsoring member state/District Parliament’s responsibility to ensure that those living in the colonies under their care have the same rights as anyone else in the Federation. For your homeworld’s circumstance, it would enable them to have their say in the governance of the Federation, as well as a mouthpiece for the needs of the population. The rights of the individual are enshrined in the Constitution, rights that are inalienable and protected through the District and Supreme Courts.
The sovereignty of the member state would also be guaranteed through the political and military protection of the Federation. Indeed, member states are sovereign entities in their own right and can choose a system that best fits into their culture, as long as it is democratic in some form (keeping in compliance with the Charter/Constitution).
Economically, the Federation is the largest economy in New Eden and interstellar trade is the lifeblood that circulates in its arteries and capillaries. The economic well-being of it’s member states are of paramount importance, with numerous Federal incentives and schemes able to assist them economically as long as consent is given by the member state. The advantages of access to this network have been previously stated by others in this thread: frictionless trade, co-operation with other member states and raising the standard of living for all.
As for the protection offered by the Federation, you are quite right to be suspect of it’s effectiveness due to the Intaki Crisis in YC121. Intaki was the victim of the state of affairs that was enabled by the Invasions of YC110, as is the case with the rest of those worlds that falls under the Federal Defence Cordon as part of the CEMWPA legislation foisted upon the Federation. That the system was under the control of the State Protectorate enabled Lai Dai to carry out their work without intervention by the Federation. If any reason is good enough to bring an end to this brushfire conflict, it would be to preserve the dignity and rights of those member states in Essence, Placid and Verge Vendor.
That said, history is replete with examples of the Federation coming to the aid of it’s member states in times of need. Sinq Laison and Verge Vendor were liberated from the harshness of State military rule during the Interstellar War by the Federation. They were instrumental in bringing an end to the war that was fought between the Commonwealth and the Republic on Assiettes Six. The Invasion of Rattilose by House Kador’s Fleet was repulsed by the Federation Navy. The coup by colonial militia officers that deposed a legitimately elected government on Vevelonel was suppressed by Federal Marines as Federal aid to the civil power. The Triglavian Invasions saw the largest deployment of Federal Navy forces for decades to meet it’s obligations to it’s signatories.
There are failures, yes. The Federation is not perfect. You only have to look at our history. Our failures are laid bare for all to see and for those to judge us on. We do not forget what we have done, or failed to do. Our failures will drive us to continuously strive for improvement to not only better ourselves at the Federal level, but also to ensure that the individual and the member state continues to benefit from their choice to be a part of this Federation of ours.
Consent is key - it is at the heart of this Union and forever will be. Ultimately the choice of whether to ascend to a member state, or for a foreign state to initiate the proceedings to join the Federation, lies with the people. Know that whilst we can make the case for Ascension, the decision will be yours to make.