It’s clear that if you want to save costs down to a penny, IB is the cheapest option, long term. Yes, you start with 20k, but I’m sure that’s not the goal, the goal is to grow.
However, if you are ready to give up on something like 100 CHF per year (I don’t have the numbers in my head, and of course it depends on the total), it would buy you a lot of simplicity to have a bank account & broker directly by PostFinance.
If you do opt for it, however, I would recommend that you buy products in CHF. If you buy VT (which is in USD), PF will apply an exchange rate that is far from ideal. If you want the minimalist solution with the fewest steps possible, go for PF and buy VWRL.CH. You will not be able to reclaim the dividend withholding tax, but it gives you an easy start and is probably a bit more reassuring.
Down the road, once you’ve got the grasp of it and once your portfolio has grown, you can run the numbers and consider switching to IB.
You can transfer a position to another broker.