A new division or competing in their biological sex are the two fairest options, upending women's divisions is not. Whether a transgender division would be feasible comes down to financing and marketing which subsequently leads to growth in demand. Transgender is nothing new and trans athletes of the past would have had to compete in the division for their biological sex. All athletes make sacrifices in order to achieve their sports goals, all athletes weigh what is the most important thing for them. So just as athletes have to choose between their sport vs. where they'd prefer to live, pursuing/ending relationships, or passing on various other opportunities, this would be another potential choice to make. Like all other considerations, it's an individual decision.
Further, per the trans community, "transgender" is not exclusive to gender dysphoria and does not requires medical remedies, identifying alone as a gender is sufficient. This means there can be trans athletes who don't have dysphoria, who have only socially transitioned. They would also be forced to medically transition if they wanted to compete against the opposite natal sex.
But one thing is clear, the answer is not to strip biological females of hard-won opportunities and destroy female records just to accommodate another category of athlete.