Your graft and tree looks really healthy. If you just want to grow the grafted lemons then leave it alone and you should be gettings lemons this year or next.
If you want to add more other varieties (madarin, orange, lime) then I would graft one on the second branch above the graft union.
If you want to graft fruits that will be larger such as the grapefruit or pomelo then I would use the lower rootstock ( 1" branch) to graft those varieties.
If you like lemons, the seedless Lisbon is good (large fruit, mostly seedless) or the new one from UCR, it's called seedless Eureka, nice medium seedless fruits. I got a huge seedless Eureka fruit on my branch this season.
Here's some fruits from my seedless Lisbon, it always has ripe fruits on the tree all year long.