I would add some observations, based on someone who has wrestled with transmission issues similarly and understands them somewhat. Not authoritative, but maybe some outside thoughts can help:
1) Have you verified you indeed have the “Gen2” transmission? I have read some models, the XC90 for one, hung on to the Gen1 a few more years, until like 2013 or so. Perhaps the diesel models could have, also? The 2010/2011 break in the transmission models was in fact not a “clean break” across all models from what the XC90 community says.
2) Some people here have indeed pondered doing what you are thinking, trying 3309/T-IV in lieu of the 3324/WS. Do not know if it has been done, but it’s been seriously considered out of frustration.
3) On fluids, as I understand it, the 3324, besides being thinner, has more friction modifier to deal with more programmed slip from the TCU. Reverting to a 3309 fluid may not be wise.
4) A lot of Toyota/Lexus people had very bad experiences with 3324 in the early years (c.2008 to 2012, say). It would work fine initially, but then shear quickly and be worn out, slipping badly by 60k miles. This was mainly in heavy trucks, and higher-performance cars. We learned to use Valvoline MaxLife for better performance and longevity. So I say this to suggest, you do have options besides 3324 you could try, depending upon if they are available in Sweden. We have a Lexus GS awd and I would NEVER put 3324 in it, based on past experience. I use MaxLife and it works perfectly at 184k miles now.
5) Have you considered doing a line pressure test to see if pressure is actually an issue? They are pretty easy to do. You connect a pipe fitting to the test port of the transmission, then a pressure gauge. You observe it while shifting manually between gears. Usual values are around 250psi peak at each shift. Values for the unit should be in the ATSG manual, or Volvo manuals. If values were much lower, it would imply TCU or valve body issues, not fluid or clutch ones. I have a suspicion this is not a pressure issue.
6) If the Lubegard was just added, give it a little time to work. The ester oil base will slowly condition seals and restore lost line pressure, if that is an issue. I have had this work in one Honda and also one Volvo (AW55 transmission). EDIT: I see you just added it. This is not an “instant” fix. Give it a little time.
7) If you are going to dump out the fluid again, a) Consider adding some solvent like SeaFoam or Chemtool B-12 to the transmission for a few days/100 to 200 miles of driving. It will have a positive effect on cleaning and also quick seal conditioning. But it’s not something you want to leave in long-term. If you are flushing fluid, that’s a perfect opportunity to do that. It can do no harm, and costs very little. And b) add some Lubegard in the new fill, again.
None of that may solve your problem, but may help think the issue through and try some low-cost/free diagnostics steps to help advance a solution.
lycka till!