Hiya! Simbi is a brilliant bun name
I had my female spayed over a year ago, and I had the same concerns. In fact, I think we all worry when our bunnies are having something so major. A spay is very invasive but providing you have an experienced vet, there’s little chance of any issues. The main thing you want to watch out for is her appetite afterwards. It’s very important that she eat up until the operation itself (do not fast her) and that she starts eating as soon as possible. Make sure you get pain relief for her, as a bun in pain is less likely to eat. I’m not sure what the protocol is for your country but in the UK, my girl was given a 24 hour pain relief injection and then I gave oral pain relief every day for 5 days. She had a very quick recovery. She was spayed on a Friday and nibbled a bit Friday night, Saturday all she wanted was romaine lettuce and by Sunday she was practically eating normal. Not all females recover as quick but if she eats and is kept warm, she’ll feel a lot better.
In regards to keeping warm, you could try a bunny safe hot water bottle or a heat pad (with no wires) Mine had no interest in the heat pad so I moved her enclosure to be in front of a warm radiator and that really helped.
Spaying her really is the best thing for her. She’ll be a bit grumpy at you but once she’s feeling better, she’ll come round. Buttercup gave me the stink eye for about 2 days.
Oh, if you are able too, I would opt for some pre-spay blood work to be done just to make sure everything is working as it should. I never did for mine but in hindsight I wish I had (although I had no issues) It might help you feel at ease too.
Good luck 