First, enlarging the rear triangle to 135mm doesn't affect the fold, the bike folds as usual and the folding width is the same.

With a special jig as done by Kinetics Glasgow, it can be perfect also for the Brompton titanium rear triangle. This was done for me by Kinetics and I mounted a Rohloff hub, its the perfect gearing with a range from 1.54m to 8.11m (19” to 102”).

For a derailleur solution, on the Birdy with about the same wheel size, its possible to use a medium cage derailleur (i.e. without risks to damage it) like the Shimano Ultegra RD-R8000GS that support cogs up to 34t.

With a 9-34t 11s cassette (what is also mounted on the superlight Helix), the range is 378%, with a 50t chainring its a good gear inch range for most cases even steep hills (I have a double 52-36t chainring with a 9-32t 11s cassette on my titanium Birdy and I almost never need the 36t chainring).
The 35x349 wheels of the Brompton are slightly smaller that the 40x355 wheels of the Birdy Touring (my titanium Birdy was factory equipped with even smaller wheels of 32x355 due to Kojak tires that have a theoretical overall diameter identical to the one of the Brompton).

There is also a 9-39t 11s cassette that provides a slightly wider gear inch range (433%) than the 12s 3x4s Brompton solution but requires a long cage derailleur like the 10s RD-T610-SGS mounted by Riese & Müller on the Birdy Touring (which is also compatible with 40x355 wheels without risks of damage).

These 9-34t and 9-39t are high quality, lightweight cassettes, the RD-R8000GS Ultegra is a lightweight derailleur, the 11s derailleur solution weight much less than the BWR+4s derailleur solution of Brompton and is much more efficient (even on the 9t cog because the efficiency of the BWR on the 1st and 3rd gears isn't good due to the relatively big reduction/multiplication ratio).