Balancing a really wacky encounter

I haven't used the forums in ages but you guys have always been really helpful and I'm trying to get back into playing D&D, not into posting unfortunately, so here's a question: I'm thinking of running a oneshot using 2014 rules for 4-5 level 2 characters.

There is only one encounter in the oneshot and it's at the end, and the characters have a small chance of getting surprise on the opponents but normally won't be able to. Furthermore, one or two players dying isn't the end of the world bc it's a oneshot, but it's an atypical encounter so I'm having trouble balancing it:

Monsters:

• Thug with 40 HP.
• 2 bandits with 15 HP each.
• At the start of the third round or after the thug and bandits are killed (whichever comes first), a Minor Spaghetti Monster with the following revisions enters the battlefield: -It is chaotic evil.

-It is medium.

-It does not speak common or pirate. It only speaks abyssal.

-Its whip attack deals 9 damage.

-Its multiattack allows it to make one whip attack AND one marinara attack.

-It has +5 to hit.

I guess I'm wondering, how should I balance this to be a hard but winnable fight for 4 players, for 5 players? Is it too easy, just good, or too hard? What should I consider changing? I made this encounter complicated with the possibility of surprise and with the spaghetti monster entering into the battlefield much later. Any help would be much appreciated. Thanks!

Maybe I should give the thug 45 HP but decrease the spaghetti monster's HP to 65?

Give yourself options.   Have a bandit arrive before the others are all dead, determine if the party is doing well or actually getting in a bad way.  Bring in the spaghetti monster and a bandit.  If it goes well for the party add a bandit, it it is going well for the monsters, retreat the spaghetti monster and leave a bandit to cover the retreat.