Wiring for kitchen remodel - appliances, electric oven/microwave combo, fridge, 2 dishwashers
Upgrade that tiny sub-panel. For under $200 you should be able to get a 30-space Eaton panel that will let you re-use all the breakers in your existing 8-space Eaton panel, plus wire and sub-feed lugs for your Siemens main panel. Speaking of which, that BR240 breaker in your main panel that feeds the pictured sub-panel is an alien - it does not belong in there. It may seem to fit, but bus stabs and breaker jaws have subtly different shapes and mixing incompatible types can lead to arcing and bus damage. If you keep the sub-panel as is, you need to replace that with a QP240.
Note, if you replace your sub-panel with a bigger Eaton unit, you can recycle that BR240 by moving it into the sub-panel and use it to feed the new oven.