Swern oxidation should give you the aldehyde without trouble. Pinnick oxidation is worth a try to get to the acid, although oxidation of the thioether might be competitive. (Chlorite is known to oxidize both substrates.)
If you don't mind adding protection/deprotection steps, it is possible to protect the thioether as a sulfonium salt:
Article Thioether protection via selectively cleavable sulfonium salts