I would go with a variation on option 2
I would notch two pieces of 2x6 pressure-treated dimensional lumber to accommodate the plumbing (vent and waste), then stand them on a 2x6 pressure-treated sill lagged to the floor with the "studs" 16" apart O.C. From there, you can then install 2x6 blocking (top blocking on edge, bottom blocking on the flat) between the bump-out "studs" and surface mount the new panel to the blocking members, thus placing the vent and waste lines outside of the 110.26(A) clear working space.
If this "mini-wall" proves impractical, there's also the option of using angle brackets and tie plates (Strong-Tie type ML angle and type TP tie plate, or equivalent) to mount notched 2x6 side blocking on edge to the existing studwork and boards for at least 48" from where the top blocking goes, and then mount the top and bottom blocking boards as specified below.
As to your new panel, I would set the bottom blocking member to accommodate a 43" to 48.5" high box -- this provides room for a 54 or 60 space loadcenter, depending on whose loadcenters you go with (Siemens has a 54 space option that is reasonably priced, while Eaton offers 60 spaces in both their BR and CH lines, and Square D offers both 54 and 60 spaces in QO and a 60 space option in Homeline as well).
As to why a subpanel is probably unwise
The NEC prohibits the classical "split bus" panel configuration with a rule-of-six main disconnecting means forming the top bus structure and a subfeed from that main disconnect powering the bottom bus structure in 408.36 Exception 1:
Exception No.1: Individual protection shall not be required for a panelboard used as service equipment with
multiple disconnecting means in accordance with 230.71.
In panelboards protected by three or more main circuit
breakers or sets of fuses, the circuit breakers or sets of
fuses shall not supply a second bus structure within the
same panelboard assembly.
As a result of this, you'd need to fit a main breaker somewhere in line with the existing panel to make it conform with current NEC standards. However, since your panel does not have a place to mount a main breaker in place of the main lugs, and there is no space above the panel to mount a main breaker in a separate enclosure, you would have to use a backfed main breaker configuration, and that poses two problems for you:
- GE split-bus interiors likely do not support hold-downs (I checked with GE tech support, and they weren't able to provide a definitive answer, but were skeptical of it working) for backfed mains, and said hold-down kit (a TQDLRK in your case) is required by NEC 408.36(D):
(D) Back-Fed Devices. Plug-in-type overcurrent protection devices or plug-in type main lug assemblies that are
backfed and used to terminate field-installed ungrounded
supply conductors shall be secured in place by an additional fastener that requires other than a pull to release the
device from the mounting means on the panel.
- The correct main breaker for your panel would be a TQDL21200 (or equivalently, a Midwest Electric CB2200B), but that breaker is a bit difficult and costly to obtain. It also would take up all 4 spaces at the top of the panel; this means that you'd have to use a 200A feeder connected to the panel's main lugs in what's effectively a feed-through lug configuration, instead of a more normal setup with a feeder breaker.
Double the loadcenters, double the fun
Adding an extra "stud" or side-blocking member to the support configuration would provide space adjacent to the replacement load center for an additional loadcenter. This could be connected in a "daisy chain" fashion, using a subfeed lug block in the first loadcenter, jumpered using fat wires to the main lugs on a second loadcenter, or as a traditional subpanel.