You could this two says: an actual siege (which takes a long while and involves psychological stress as well as fighting), or Call of Duty Zombies (waves of enemies).
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If you're interested in an actual siege, I had a somewhat similar scenario in an adventure I wrote, The Dying Skyseer, where the party goes to an abandoned church built into a cliffside, to meet with a witness to a crime. Beforehand they learn that the church has a small blessing over it to stop bleeding, which means people reduced to 0 HP auto-stabilize inside.
Once the PCs get inside and meet the witness, some attackers sneak in and try to ambush them, but are probably repulsed. The leader of the attackers can phase a limited amount, so once he gets below a certain threshold, he phases as a reaction and tries to flee outside.
If the party tries to leave, they find the door to the church has been blocked off by thick metal bars that were telekinetically slammed into the stone walls of the cliff. There's a small courtyard outside the entrance, and the attackers have a few other minions who are hidden behind cover with ranged weapons pointed at the door, just in case the party tries to pry the bars out. As far as the attackers can tell, they've sealed the PCs in, and now they're sitting ducks. One of the minions outside tells the party to sit tight, because his boss is coming to offer a trade.
In truth, there's a couple ways to sneak out, of various ease and risk. The witness knows about a narrow chimney which will emerge higher up the cliff, so getting out that way is probably not something the whole party can safely manage, but maybe they can send someone to get help. There's also a latrine that could be crawled through, which exits behind some bushes that are within sight of the courtyard in front of the church, so the party would need to be sneaky not to get spotted.
A few minutes later, the mastermind badguy shows up in a carriage and says that if they hand over the witness, he'll leave, and then the PCs can get out of the church safely. This is a lie. The mastermind wants the witness, sure, but is also okay killing him and the PCs.
An hour later, a second carriage shows up with a hostage - one of the PCs' acquaintances, whom the mastermind says he'll execute if the PCs don't hand over the witness. If the PCs agree to a swap, the witness goes out and the hostage goes in with the party, and then the minions telekinetically put the bars back in place. If the PCs don't cooperate, the mastermind executes the hostage.
And regardless of whether the PCs hand the witness over, the mastermind doesn't leave. The mastermind's minions show up with explosives, and they slip up in blind spots to place the explosives above the church's entrance, to collapse the cliff and seal the party in. After the explosives are placed and the fuses are set, the villains all retreat, and the party has about a minute before the boom.
What usually ends up happening is that the party realizes it's dangerous to sneak out and wait for an opportune moment. Then they lose their friend, gets furious at the villain (which makes the payoff for beating him later more satisfying), and then rush out the latrine tunnel as the bombs are just about to explode. Some PCs rush out and try to rescue their friend, at which point the minions fight them and the mastermind has a magic item to let him teleport away. Some PCs just find a reinforced area in the church and tank the cave-in (and they auto-stabilize due to the blessing), then wait a day to get rescued by their friends.
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Your situation sounds different in the challenges the party faces, and because the party chose to hole up, but the same feeling of being trapped exists, and let me tell you, PLAYERS DO NOT LIKE FEELING TRAPPED. Seriously, in a game with a lot of power fantasy stress relief, feeling like they have no good options makes players wail and gnash their teeth and do suicidal stuff because they think GMs are being unfair. We playtested the adventure and made some key changes to ensure the players knew they had several ways out of the situation, and it was a matter of whether they can pull them off, not whether they can find out that they're options.
A good siege isn't just a series of attacks you repulse, but a mix of that and dilemmas. When the attackers realize they can't just overwhelm the party (because there are choke points), and the party knows they can't reasonably make a break for it without getting cut down, the two sides will want to break the siege. Put some stuff inside the diner that they could use to lay traps or maybe build something to cover their escape. Which tactic will they use?
Drop clues that there's a secret entrance (like a sewer tunnel), but don't make it 100% obvious, so the party feels clever when they piece the clues together. Either they can flee through it, or the attackers might discover it and ambush them.
Have someone try to negotiate with them. Is there anything the PCs might want?
Maybe bluff the PCs. Perhaps they see a 'shift change' in the besiegers, and it looks like a good time to make a run for it, but some hints make something look off, because it's actually an ambush to try to lure the party out.
Someone comes to try to set the building on fire. Can the party stop them? Or they create a fog cloud to hide their approach. Or maybe the party spots a flickering light behind the baddies' line, and they realize one of their friends is sending them a message.
What's the 'win condition' here? Just kill all the attackers? Do a ritual to teleport out? Find some hidden clue in the diner that they need and then get the f*** out of Dodge?
