Generator interlock. That was easy
First, if you just want a manual transfer switch, these can be easily installed as a generator interlock in the vast majority of panels. This is simplicity itself: install a breaker in a particular location in the panel and backfeed it. Install an interlock, a simple see-saw mechanism that assures only one: the generator breaker OR the utility breaker, can be on at once.
Such interlocks typically cost under $100 (as low as $30 in Siemens' case).
Otherwise, install a second meter pan
Use a modern "meter-main". Also install riser, weatherhead and service entrance cabling. Then, call the power company and have them move your service drop over to the new weatherhead.
Here's the problem. The power company won't install this unless they see that you pulled the proper permits from the city and got everything inspected. In turn, the City will want to see what you're doing with your new panel. So now, you're "in it" for installing a whole new service panel, *and having the house 'down' while all this happens. We're into thousands of dollars at this point, much of that in hotel bills!
"Generator subpanel" is the least bad option
assuming you want an ATS. Because the generator subpanel doesn't require the power company to get involved. You can move circuits one at a time, at your leisure, without taking house power down for days (most of which will be sitting around waiting for inspections and the power company).
This also permits a smaller generator, since a generator under an ATS must be sufficiently large for all the loads to be served. With a generator subpanel, you can pick a subset of your loads, so you're not having to install a yellow Cat generator only because of loads you're not even that attached to.