Thank you kindly for your thoughtful reply, and for letting us know how things stand in working toward a solution.
Re: "I asked them if 4010 was particular to Sequoia 15.7.2, and have not heard back yet."
As Sequoia 15.7.3 is the newest Sequoia update, they may be working with that version now, (especially as it contains security updates).
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Re: "I usually wait a few months before doing a serious upgrade like 26. It gives appsDevs time to adjust".
Wise to do that, if waiting to use new features, (or any increases in privacy and security) is not a concern.
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Re: "Even if I backup my Mac to TImeMachine and then install 26, I cannot go back to Sequoia, correct?"
Correct. Seems technology always moves forward toward improvement, and, at Apple, in keeping privacy and security of our data up-to-date.
Thank you. All the best :-)