Thanks for the question and glad to hear that you enjoy using your TS. I suspect you'll get some advice from our membership here on the forum, but generally the Rotex 125 and 150 are considered the best all around sanders because they can handle aggressive stock removal (similar to a belt sander) as well as finish sanding.
Here's good resource on our website that is a decision tree for choosing the right sander. Hopefully between that document and the feedback from other members, you'll be able to make a good choice. If not, remember that we offer a 30-day money back guarantee if you find the sander you choose doesn't perform as expected or meet your needs.
I started with the RO150 Rotex.
It can come close to doing the work of a belt sander & do final finish sanding & most everything in between.
Then I added other sanders over time.
I just sanded down a total of 30 m2 wooden ceiling using the RO125 and grit 60 paper. Combined with the CTL Midi dust extractor I must say I'm impressed with how well Festool handle dust removal. My arms may have been sore but the floor was clean.
I would definitely recommend checking out the RO125 or RO150.