I may not be a trans expert, but I am good at sleuthing.

First off, if you haven't found it, be sure to check out the FAQ feature article here on AW7x Rebuild
https://www.brickboard.com/FAQ/700-900/TransmissionAutoRebuild.htm
also the one here on valve body service
https://www.brickboard.com/FAQ/700-900/VolvoAW7XValveBodyService/AW7Xindex.htm


Here's a link to a Volvo Service Manual that covers the AW71L
http://www.krona74.ru/library/Auto/Automatic transmission aw70-72l.pdf

Here's an earlier Volvo Service Manual that may also be of interest that covers up to the the AW71 trans, extremely similar to the AW71L except for the valve bodies and torque converter being slightly different
https://www.240.se/litteratur/tp30868_2.pdf

Any AW72L manual you find will also be useful. Those were used with Volvo's B234F 16-valve engine of similar vintage. Only the lower gearing and valve bodies were supposedly different.

There was a good AW71 rebuild discussion over on Turbobricks here with good pics and comments related to such failures as yours.
https://turbobricks.com/index.php?threads/late-model-aw71l-disassembly.243382/.

An AW71L rebuild kit can be found here
https://www.oregonperformancetransmission.com/product/OPT-AW71L-RB.html
It doesn't say it's for the 900s, but I don't know why it wouldn't be. At over $300 it's not cheap. If you can't find a cheaper source then a good used one may be a better option when you consider the time saving and the odds of doing it right as a first time effort.

Toyota used these same transmissions, so follow any links you find into Toyota user groups and references.

It's a rare day on the brickboard when we hear of a serious trans issue such as this, not just sluggish behaviour. I'm crossing my fingers for you that it's the fill level, but based on your comments of a run-in followed by a top-up, I can't say I'm overly hopeful if you filled it to max, not just min. Smells like valve bodies to me, but again, I'm no trans expert.

Please keep us posted of your progress.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now