could mark be a type of scar that shows their own experience and how everyone has experienced something traumatic in life at a point

The repetition of ‘marks’ is using a method called antanaclasis, repetition of a word with a different meaning in each case
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Lit Genius (Ft. Lit Genius) - Glossary of Literary Terms — Genius

A / Acronym / Acrostic / Adnomination / Adynaton / Alexandrine / Allegory / Alliteration / Allusion / Amphimacer / Anachronism / Anadiplosis / Anagnorisis / Anagram / Analogy

“Every face” suggests the current situation affects everyone. Exaggerated. Hyperbole.

the word wander, could imply that the character is trying to find a purpose behind everything

The word “I” implies the poem is going to be narrated in a First-Person Narrative, which can personalise the poem and even make the poem seem more realistic than a third person narrative poem.

Blake also lived in London himself (Soho), so he is writing as a resident, not a visitor. He is sharing his own experiences and the horrors he witnesses on a daily basis, wandering through London. Perhaps the fact that he is a resident but still seems like an aimless flaneur suggests he feels lost in the bustling, miserable streets, despite knowing them well, and unable to recognise beauty in the controlled, industrial society of the late 18th century.

@frazza You’re right that it is the first person singular, but the persona, probably the poet, isn’t an upper class gentleman. Blake was intelligent but could afford limited education and worked full time as an engraver, often in financial difficulties. He is an observant, intelligent and compassionate poet/artist, not a remote member of a privileged class. His sympathies are entirely with suffering, abused poor people.

Saharia. It is a first person singular beginning, not third person.