Pachira aquatica - Money Tree - nice save from root rot. I'd let it grow out for a couple years. You need to get it big enough to make a few more cuttings. The tree has meaning to your girlfriend, ''Doing Bonsai'' on many tree species always has the risk that what you will kill it, you need an heir and a spare before starting bonsai techniques or risk having an upset girlfriend.
Pachira is a tropical tree, where you live you will only get strong vigorous growth during the summer months, do all bonsai work between May and August 15, while the tree is growing vigorously. If you can put it outside for the summer. Rest of year, keep in sunny window, water frequently enough to keep moist and fertilize it at least once a month, or once every 2 weeks. Keep fertilizer dilute, but don't skip it entirely.
As a species of tree for bonsai, Pachira has many traits that make it difficult. Its leaves are large and compound. They will reduce some, but not as much as you would wish for in most compositions. The branch structure is coarse, you will not be able to get fine twigs as you develop branches on your branches. A good trait is rapid growth, a bad trait is rapid growth (hard to keep a shape). Growth habit is very upright, it will fight every attempt to make it cascade. A cascade is defined by branches extending below the bottom of the pot. A semi-cascade is defined by branches extending below the upper rim of the pot, but above the lowest edge of the pot. A true full cascade always has to be on a stand, or branches will be bent by the shelf or table it is on. Pachira will withdraw support to downward branches, and push all support to buds above the rim of the pot. Difficult to keep a cascading branch healthy.
Because of leaf size, If it were mine, I'd go for a medium large house plant with an attractive shape, and forget about bonsai with this plant, especially if this is your first try at bonsai. If your really want to turn it into bonsai, you could treat it as a bonsai stand in for a palm tree, each compound leaf positioned to represent a frond of a palm.
For tropical bonsai (indoor bonsai) I recommend Ficus, Eugenia, Malpighia, Portulacaria afra, and florist's azalea (R simsii hybrids, not Satsuki hybrids). Florist's azaleas are best for homes that run cooler than 70 F in winter and has naturally soft water (low dissolved solids) available. The others do better in warmer than 70 F homes. There are hundreds of tropical and subtropical trees that can work as indoor bonsai, but the ones I listed are pretty easy. (except maybe the azalea - it can be easy, if water & light are good)
So those are my thoughts, hope this helps.