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Hi there !
I'm not sure adding another sound source is what your rack needs TBH - even if I would wholeheartedly recommend the TsL as it's a really great little VCO, quite powerful for its size - you already have 3 !
Personnally I'd add a filter - for example I just acquired the new Grainity, and it's amazing. It can be a filter, a sound processor, or both, depending on how you use it.
Another good candidate would be an O&C, the Phazerville firmware really brings it to a "desert-island" level module IMO, and it brings enough new functionality to your rack to stay relevant even having Pam's in there already.
There's also matrix mixers, they're often recommended for a good reason, AI Synthesis offers theirs as a kit, a very nice first soldering project if you want to try your hand at DIY.
These are mere suggestions based on my own preferences though, ultimately your question can only be answered by yourself: what are you currently missing when you patch ? Why are you thinking about the modules you mentioned (TsL and Ochd Expander) ? Answering this will help others help you ;-)
Take care !