One approach: rebuild the rear wheel with a DualDrive hub.
it is a combination of a cassette with multiple gear cogs on it,
and a internal geared 3 speed hub.

Sachs, now owned by SRAM, made a 7x3 years ago,
now that cog stack has gone up to 8 and 9, the 7 speed discontinued.
and this year Sturmey Archer has added a similar product.

you will have to spread the rear end some,
though maybe not much since you have a derailleur
on the bike already..
8~9mm spread / 126 ->135 4,5mm per side.

then the gear range further adjusted by the size of the 1 chainring in front..