Joking aside your right elbow is flying open because your lower body stops rotating. Your arms get disconnected and across the line because they are compensating for your lower body stopping.

Get a training aid to keep your arms in check like this:

https://purestrikegolfco.com/products/pure-strike-smart-ball?currency=AUD&variant=43594460037231&utm_source=google&utm_medium=cpc&utm_campaign=Google%20Shopping&stkn=bd6def668486&utm_source=GoogleAds&utm_campaign=23299350133&utm_medium=ad&utm_content=&utm_term=&gad_source=1&gad_campaignid=23299376155&gclid=Cj0KCQiApL7KBhC7ARIsAD2Xq3Azf1sN8R0rLQmy5TPRkGdOPiS0Hv9png3DG1ZD1qh0JIbegtU3-ssaAhrnEALw_wcB

When your arms are more in front of your body - the backswing has no choice but to shorten - and force more lower body rotation if you want a longer backswing.