Are the lemons of equal quality on all the stems? I see spines on one stem which implies that it is from a seedling and is maybe a rootstock. In that case that stem could outgrow the others. But if the fruit is the same on all stems why change it? It's only a matter of aesthetics.

Yes unless you have citrus grown from cutting some or all of that growth could be rootstock. I would ID and remove rootstock and see what you are left with. You can grow as single trunk or multi doesn’t matter so much