O.k. Here's my take which is likely to make me unpopular with either side but it means I'm probably on to something. And I say this as someone with a bit of background in this situation being LGBTQ myself, and having a relative who is T.
Firstly, sports is never fair. My son is August birthday and a GK. He was the tallest in his age group, but when they moved around the age groups he has to compete now against kids with 6 months growth on him. He's in the 75% of height for males, so not a short keeper by any means, but still that 6 months makes a huge difference. He also wound up losing a year of development when they changed the years. Similarly, if there are any girls on your teams with a mix of chromosomes (like the xxy) , or if they just happen to be genetically early bloomers and towered over their teammates when they were 10, or even if they are over 6 ft, there's nothing fair about that either.
What's "fair" or "unfair" here is entirely in the eye of the beholder. Any fidgeting of the rules will produce winners and losers, since there are finite places on a team. It's not "fair" if your DD gets cut from their dream team because somebody who looks like a guy walks in from tryouts and just blows away everyone even the strongest girls. It's not "fair" either to send the trans kid who has behaved as a girl since they were 8 and is on hormone blockers to compete against 18 year guys. Start from the position there is no "fairness" here. Also start from the position that I have yet to see a dude decide he wants to compete in women's sports just to gain a scholarship or trophy (though it also wouldn't surprise me to eventually see one or two).
For 10 years or so, before the woke knives came out, we had the NCAA standards. They, IIRC, required an athlete to be on hormones before being allowed to compete with the other sex. They worked for a long time before politics overwhelmed this issue.
Knowing that no solution can ever be perfect, and understanding we have to try to take into account the concerns of all participants, I'd think a siding scale is the most appropriate scale. For youth rec, who cares. I once had to referee an AYSO game with a kid who was MTF, 9 years old or so, playing with the girls. Very sweet kid, somewhat in love with the attention being given, and could not see that child playing with the boys and not getting hurt and/or bullied. For youth club sports, it's tricky because it's at an age before hormones are advised, but I can see a rule where hormones and/or testosterone blockers are required for some length of time, but the rules should be less stringent since it is a team sport and the contribution of any 1 athlete is limited. For individual youth sports, because the performance is so dependent on the performance of an athlete a stricter rule is called for. For college athletics, the rules should be even stricter and require a prolonged period on hormone blockers and estrogen, plus either a transition or an active moment towards transition within the coming months For Olympic level competition, it wouldn't be unreasonable to require the transition to have occurred.
None of this is entirely "fair" to anyone. But it recognizes there are competing concerns on both ends of the spectrum. Whether or not a transition really makes someone a "real boy" or "real girl" is besides the point here...who cares....it's all about definitional nonsense either....and no a transition does not make you magically into a biological female. We need to balance compassion on one end of the spectrum with fairness on the other, knowing that "fairness" is impossible to achieve in sports. We were headed in that direction before politics inserted its dirty nose into this issue.
