You should definitely state on the website that version 25.0+ is required for creating the Safe (ideally in the combo-box where you can select the type of Safe you want to create). It was the only way I was able to sign the PSBT.
note: this allows you to sign PSBTs from Safe wallets created in older versions of Core
Yes we still need to improve the flow a lot, that's why we are testing and doing reviews. Thank you so much