You mentioned you see broom style here. There is no real need for trunk taper in broom until after the first fork so I don't see any point in recutting the large cut.
I've made the left trunk a little longer because I like some asymmetry, even in broom style forks look good when they are not all at the same level.
Cut at blue line. When new shoots grow keep 2 or 3 growing from each cut - green lines. They will be the primary branches for your broom canopy. The usual process is allow the initial branches to grow long to gain some thickness then chop again a little higher to get further branching. Repeat until you have the required canopy.
Please note that I have never grown Siberian elm but I understand they are very easy to grow and respond well to chopping like this.
For a shorter broom tree the red line above would probably be a suitable start. There are nearly always alternatives in bonsai.