その上・更に: the difference
その上N2
Meaning
Besides, In addition, Furthermore
Past papers
Example
彼は勇ましい。その上優しいのだから、言うことない。
He's brave. On top of that, he's kind — I couldn't ask for more.
更にN2
Meaning
Even more or further, Again, More and more
Past papers
Example
税金がさらに高くなった。
Taxes went up even more.
How to tell them apart
「さらに」: formal; can mean 'more' in degree or 'in addition' in information.
「もっと」: casual; degree only, often what you yourself want more of; it cannot start a sentence to add information.
「その上」: only adds new facts; it cannot modify an adjective for degree.