This is a flight departing the EU, so EC 261/2004 applies. While there is no compensation due because this change was made more than 14 days prior to the flight, it is incorrect to say that OP's only option is a refund.

EC 261/2004 permits the passenger to select from the following: a refund, rerouting under similar conditions, or rerouting at a later date selected by the passenger.

You will likely need to escalate this at GA and you should be polite but firm on GA's obligations. While there is no precedent for requiring a carrier to rebook on another, the CAA has recently threatened FR with action for not offering to do so. The facts in the NR matter are vastly different than this, but it bears some note.

You should have written down the various options in the order you wish them and push hard, mentioning the legal obligation under the Regulation.