I’m hoping to run an evil campaign, where the players (starting at Lv. ~2) have to escape a prison demiplane, and I’d like some input.

I already have some ideas, like the guards having special armbands to cast Banishment (to return home) but only work once every seven days, and a Wizard inmate who can cast it on his own but has been drugged up to dumb him down and prevent him from using more powerful spells. I’m also thinking about including classic crime elements, like prison violence, racial profiling, and a drug-smuggling story element.

Like I said, I would appreciate some input.

Is escaping the prison supposed to be the campaign, or just the "you all meet in a bar" equivalent?

Yeah, it’s the “you meet in a tavern” trope but as a prison gang.

In the event of an unsuccessful escape: Suicide Squad.

Be careful of that campaign concept. Having been a a campaign like that, I would suggest that something happens that facilitates their escape during (or at the end of) the 1st session. It’s unfortunately a fact that many players will prefer to “suicide by Townsgiard” their PCs rather than spend any time behind bars, even if it’s only a brief stint . One campaign I was in, half the players left after the second session, and the whole thing broke up after the third because we were still in prison. So let them GTFO ASAP is my advice.

The first chapter of the "Out Of The Abyss" campaign is about escaping from a drow prison.  It provides a few means to escape but ultimately it is up to the players to put their heads together and work things by piecing those items together for their master plan of escape.

Examples: 

Acquisitions

One or more of the characters might have useful items in their possession (see “Scavenged Possessions”), and working outside the slave pen creates new opportunities for the characters to acquire and hide small items, including makeshift weapons or tools, or even lift a key to the slave pen from a guard. What they can acquire depends on the work they do and where they go. Use the description of the different locations throughout the outpost as a guide to opportunities. Taking something without being noticed requires a successful Dexterity (Sleight of Hand) check contested by the Wisdom (Perception) checks of any active observers, or using an observer’s passive Wisdom (Perception) score as the base DC. A prisoner that fails the check is commanded to relinquish the item, on pain of death.

What equipment and treasures the characters claim during their escape depend on how much of the outpost they are able to explore before fleeing. For some characters, it might be a fun challenge to escape into the Underdark with little more than the clothes on their backs. For others (including spellcasters who need spellbooks or components), consider placing the party’s captured equipment (normally in Ilvara’s quarters) in an alternate location if the characters are intent on escaping without exploring all of the outpost, such as the elite drow barracks (area 4) or the armory (area 10).

It's good to have some things open/sandbox for them to be creative but you may want to layout various means they can discover to create opportunities for escape.