Can I show upsell products as a tab next to the description instead of a block?

Yes. In Plugins → Up / Cross Sell, on the Upsell tab, set Display as to Inside Product Details (as a tab) and save.

Then turn on the You May Also Like (Upsell) section in your product template's Product Details block. If you keep Hook (place on the product page), use Placement to move it higher, for example Product page — after summary.