The interesting issue with St Pancras is that it's also very overbuilt for what it is. If you look at some other past and present examples from Europe, you can see that there's really no need to have immensely elaborate departure halls for travellers as long as you have space to keep them until the train is ready for boarding.
A good example is with Terespol in Poland. The international station for departures is nothing more than a small ticketing/waiting area, an exit customs check and an exit passport control check. You then go down some stairs and up into the departure platform, where you wait for the train to start boarding. There are no shops, no duty free shops, just toilets and nothing else. The same story is with Brest in Belarus, where the international hall is just a large hall for customs control, booths for passport control, then a corridor with a duty free shop and a set of doors to the departure platform. It's enough for 200-300 people, and the departure platform could probably accommodate 500-600 under the roof if needs be.
What should be the main focus in St Pancras is speeding up the whole departure process so that they can get people to only enter the complex at T-30 or so. With the ETA, API and ETIAS, the vast majority of travellers should not need anything more than a single scan of the passport on exit at an e-gate, and security should also be improved so that there are no tailbacks there.
I think it's also worth remembering that Schengen does not impose hugely bureaucratic requirements on border stations too. For example, when Bratislava-Petrzalka was in operation for international border control, the Austrians did the control on the platform. Likewise, until 2023, Slovenia carried out entry and exit Schengen controls onboard local trains without any drama, the train simply waited at the border station. Yes, StP is much larger, but the processes themselves are not particularly problematic in the UK->France direction.