This is not a trivial task. You can start by studying this article https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3430791/

You can benefit from this article

https://doi.org/10.1016/j.tet.2006.01.118

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