I enrolled in SMFA"s diploma program after undergrad with the same intention - to develop a portfolio and eventually apply for an MFA. I already have an art minor so thought it would be a somewhat straightforward process. In hindsight, I wish that I had three very crucial pieces of knowledge before signing up:
1. The SMFA is very small and pretty disorganized. While students were making interesting work, there was as many students kind of floating along which is very easy to slip into based on the environment. The Diploma program itself is pretty flexible, which sounds great initially but what it means is that you will not receive overall direction. You will most likely get a sampling of a lot of areas and mediums but it will be up to you to sort through it all and map out a cohesive track. I floated around for a bit before realizing that I was necessarily developing my hand in a particular area. Classes at the SMFA are also extremely expensive, so I would think carefully about how you want to invest your money. To me, it's more of a money-making program than anything else. Humanities courses are not part of the curriculum so you are not technically allowed into art history courses which are as important to take in your development as an artist. Many of your fellow Diploma program students are retirees and some of the BFA students will look down on you for being a Diploma student.
2. Art teachers, in general, are not necessarily the best at giving you sound advice in regards to academics. Two professors there gave me completely inaccurate guidance and told me that I couldn't earn another bachelor's after earning my first degree in a separate field. This was backed up by an academic "advisor" who was someone I only met with once or twice throughout my time there. I did a little more research but couldn't get a straight answer from anyone because they honestly didn't know. It was only when I starting working in admissions at another art school years later that I learned the guidance I received was inaccurate. Honestly, if I had switched to the BFA program back then, I would be in a better place professionally. Make sure you talk to multiple, knowledgeable sources and do your own research in regards to mapping out your academic path. It also blows my mind that there wasn't anyone in the various departments that could accurately answer such an inquiry.
3. Massart next door offers far more affordable studio art courses and greater resources for students who want to dabble in a lot of different areas before focusing in on one in particular. I ended up earning my Post-Bac from Massart after realizing that the SMFA Diploma program was not very structured and did not necessarily offer a clear path towards an MFA. Again, I would have done things very differently than what I ended up doing. If I were to go back in time, I would have taken courses through Massart and then considered earning a Post-Bac from the SMFA. I did get accepted into SMFA's Post-Bac program which was far more structured and in-depth than Massart's program at the time. However, I believe the price tag was more than double Massart's tuition. Perhaps what you can do is take studio courses at Massart and then apply to SMFA's grad program.
The general difference I observed as a student at both of these institutes is the curriculum - SMFA is very much focused on theory and concept and Massart is focused on technique and craft. This is an extremely generalized statement but I found that my ideas were very accepted at SMFA and seemed to confuse my advisors and instructors at Massart. On the other hand, the students at Massart had professional-level technical skills though not always driven by the most highly intellectual concepts. Again, this is a huge generalization.
The most important thing is staying centered and focused. I was at both schools when I was relatively young and sometimes let myself become too influenced by the critique of my teachers, which were often in opposition to each other. Good luck with figuring out your next steps, and apologies for some of my grammatical mistakes, I am too tired to proofread and edit.