I think it undermines the conscious parenting effort of multilingual parents to say 'just speak to him'. Of course speak to him, but it's much harder work if only one adult around the child uses certain language fluently, it reduces the benefits of immersive learning.
What is your priority, OP? Because from what you're saying, it's sending your child to a Montessori nursery - great. Helping his social development - great. Exposing him to your culture - great. Putting pressure on him to be academically brilliant - definitely not great and it sounds like you've had it forced upon you which contributed to the way you are now. You need to recognise this and work on yourself, there are no shortcuts here.
What you are actually doing though does not in any way align with what you appear to want for him. You tried therapy, but with no success so you came back into your anxiety bubble and now you're defensive when many of us pointed this out. Your child will soak up your anxiety which shines through everything you posted on here - and no doubt through your behaviour towards him. This is far more damaging than you can imagine. I think you're not fully understanding the damage that has been done to you. Once you do, you'll also truly understand why you need to break the cycle.
In 2-3 years time you won't be able to take him to extracurricular activities that would suit him best, unless you're in central London. In fact you are probably already limited in baby and toddler groups and in outdoor activities you can take him to. Do you have any mum friends? Do they live nearby? What kind of socialisation do you offer to your son?
If linguistic immersion is important to you then you won't achieve it by being the only speaker and sending your child to a British childcare placement at the same time, that's contradictory. If that's your priority then he either stays with you full time and you speak full time, or you send him to childcare part time and up his exposure amongst other Mandarin speakers - again, any local mums with children? International schools going forward? Social groups with Mandarin speakers? If this is not your key priority then do your best yourself exposure wise and move forward, you have no control over how good he'll be, accept it and focus on prioritising other things if that's right for him.
He doesn't need to be a copy of you. This is not the goal. He's his own person and dare I say it, he might not be very smart - and you as a parent need to support him regardless. It's too early to say how smart he is going to be and what his strengths and weaknesses will be and that's not something that you can control, only support.
I understand people commenting about Waldorf, but this is dating back over 10 years now. Many things have changed since, as acknowledged by Waldorf and I personally don't think I'd be overly concerned about a couple of days a week in a Waldorf forest school. Forest schools, nature and learning through free, outdoor play are excellent activities for toddler development which you appear to underestimate because of your own upbringing. Again, there are more ways to live and learn than one that can be beneficial, there's no need to be so rigid.
In conclusion, you are trying to square a peg, you know what you want for him but you can't achieve this. You're then looking for second best but you're not even truly open or appreciative of it. If you don't remove obstacles and preconceptions existing in your own head, you'll have these dilemmas all your parenting life and it's no way to be, you'll burn out very quickly.