I’m not planning to size up the pot for my umbrella plant, I’m happy to keep it at this size and let it fan out. However I’m not sure if the branches will just get longer and less manageable (they’re certainly longer than a few months back). Do I need to start training it or winding the branches together if the small pot won’t stop the growth?
No, don't wind them together - the stems support themselves, and you will restrict air flow if you try to tie them together. The likely cause of the lateral shoots getting so much longer, longer than they usually do, is insufficient light - these plants like bright daylight, but not direct sun other than a little early morning or late evening. Over time, this plant will get quite tall (mine is currently 5 feet), but if it gets too large for your liking, you can always cut off about 8 inches of stem from the top, strip off the lower leaves, pop it into a bottle of water and wait about six weeks, when it will produce roots, and you can pot it up and start over. In the meantime, place it somewhere it gets more natural light.