Touch wood my kids aren't travel sick, but a friend of mine who has two children forward faced her eldest due to travel sickness and regretted it, so persevered with RF her younger one who also gets travel sick.
She has found that with a more upright seat (they have Axkid Minikid) it has helped. They got the summer cover for easy removal and cleaning. She uses a blind on the side window to give the child a clear view out of the back, keeps the car cool, and avoids feeding her little one too soon before any car journeys, particularly avoiding dairy products. It has helped and she reported no sick for over a year (again, touch wood!)
Good air con can really help if it's the kind which reduces the temperature inside the entire cabin. If your air con/fan is not this powerful and is getting blocked by the rear facing seat shell, then you could look into the "noggle" which is like a tube/hose you attach to the vent and then direct to point at the rear facing child.
There are currently no seats on the market which can RF and FF to 25kg. The Diono is discontinued and Britax changed their recommendation for the TWE when they re-certified it under R44.04, and say it's only suitable for FF to 18kg now. They still have the Multi-Tech 3, but this is only a booster seat after 18kg.
Do you currently have an infant carrier for the younger one? If so, it probably is worth trying a 25kg RF seat for the eldest. If he does not get on with it, then at least you could pass this on to the younger one in a couple of months and swap your eldest to forward facing. OTOH at three the difference is not as stark. It does make a difference, it makes a difference at any age, but the most dangerous period for forward facing has passed by age 3.
Does the car you're getting have top tether points? Have you already chosen it? I ask because all of the 25kg forward facing ones do need these tether points now.