Well this was a bit lengthy read.. [smile] took me almost 2 hours to read in between watching football. [eek] Let's just hope I remembered all of the pertinent points. [tongue]
Just as a baseline, I own and use a Rotex 125, an ETS 125, an ETS EC 125 and a Rotex 90. I also use other Festool sanders but that's not pertinent to this conversation.
As you've described your potential frequent sanding needs, I don't really think you need a Rotex unit. You're not removing multiple layers of paint or catalyzed finishes from a deck or sanding off a coating on cement. You're simply trying to remove an oxidized/weathered coating (probably oil based because of its age) from wood. I'd first look at the grit size you're using and then at the type of grit you're using. Both will have the largest impact on your sanding success. Using 320 Granat on a RO 125 will not remove material as fast as using 36/40 Granat on an ETS EC 125.
You mention wrist issues...the RO 125 will not help that situation in Rotex mode, and that's the reason you purchased it, for the Rotex mode. I have NO track record with a RO 150...but I think there is some spillover here.
You mention that the 150 sander seems too large on some of the projects you may be using it for, I concur, that's the reason I went with the 125 series.
My recommendation would be to purchase an ETS EC 125 along with a 150 pad. This probably voids the warranty and that may make you uneasy. However as you've already stated, all 3 sanders appear to be "basically" the same (correct) and that should alleviate some of your angst. The 150 pad WILL fit on a 125, the 125 pad WILL NOT fit on a 150, this was by design. I also believe the 125 is $100 cheaper than the 150, so you can have the versatility of 2 different sized sanders at $50 less than the price of a single ETS EC 150 sander.
The ETS ES 125/150 sander is fairly aggressive for a finish sander, it's my go-to sander every day. I find that after purchasing the ETS EC 125/150 hybrid, I tend to gravitate to using the RO a lot less often.
I'd say try the EC 125/150 combo and see if that doesn't probably meet 70-90% of your current sanding needs. If you need something more, re-evaluate and maybe consider the RO 90. Although it can also act like the local "bully on the block" from time to time.