Is placing a mark at the end of the blank page in the Mushaf after each completion of the Quran, with the aim of encouraging continued recitation, considered showing off (riyaa') in Islamic law?

Concealing good deeds that are not commanded to be displayed is legally مطلوب (required/desirable), as it is further removed from ostentation (riya') and more conducive to sincerity (ikhlas). We do not advise photographing and publishing these deeds, for Allah Almighty nothing is hidden from Him and no reward is lost with Him. Furthermore, it may lead to conceit and overestimating one's deeds. Nevertheless, photographing and publishing them is not forbidden if no one sees it and if the intention is for encouragement and invigoration.