These are excellent points.
And I first want to approach it from the "good faith" point of view. The past 20 years have seen a sea change in what surveillance and devices can do. And as such we (society at large) needs to adjust our understanding and approach to this new technology.
Firstly the opportunities are immense. With almost (*) every person having some kind of heart, lifestyle, monitoring and measurement we could see transformational medical and epidemiology breakthroughs. On top of which access to (free, correct!) information, hell GPS is amazing. We know
Secondly there are opportunities for abuse, and there is justifiable concern about privacy, about affect on democracy, truth etc. I personally think these are..the wrong terms to use, but never mind.
I think we have to assume Good Faith from the EU here. They don't know the answers any more than the rest of us do. But they have been upfront abut tackling obvious big problems - the GDPR, for all its many faults was first, and a again big Good Faith step in right direction (pace all the stupid cookie warnings)
As for answers
Which devices apply - which ever ones are "owned" by a Gatekeeper (45 Million users / 7Bn turnover).
Should Sony open up Playstation. Yeah basically. Xbox too. Will that cause problems - I expect so. In 99% of cases a big warning saying "you are side-loading this is dangerous" will prevent most horrors.
I expect this will not lead to an Open source free for all. I expect there will be develop licensing programs and approvals - because I certainly see the walled garden of iOS as a real benefit.
The bank transaction signing thing is interesting. The definition of a gatekeeper (Article 2 in the Act) is fairly specific to things like online search engines, intermediation services, OSes etc. I think its unlikely 45 million users is a big floor for such a thing.
(#) 4.5 Bn smartphones are in use globally, that's almost every adult. In "western" countries there are 10s of millions of people with daily heart rate monitoring.