Yes, you'd need to get quite far from the Sun to use it as a lens - about 650 AU is where it starts becoming practical. It would also not be re-orientable, so any "telescope" launched to that location would only be able to observe a single target. (But, notably, it is far easier to send a probe to a far point in our own solar system than it is to send it to another star entirely.) There's a paper that goes into much more detail at
https://arxiv.org/pdf/1604.06351So why not use interferometry instead? Well, it has some significant drawbacks. For example: the Event Horizon Telescope used radio telescopes - and pretty much had to, due to how interferometry works: you need to be able to compare the phase shifts between the multiple telescopes, which means you need to be able to sample the signal faster than the radio frequency you're using and record it. The EHT records 64 gigabits per second for each telescope, and then all this data needs to be combined to compute the resolved image. This amount of data would be problematic for space-based telescopes - even on Earth, it was not practical to send multiple petabytes over the internet, so it was saved to hard drives which were shipped by truck instead. This isn't practical in space, so you would need to transmit the data by radio, which means you'd end up with some crazy ratio of thousands of hours of transmitting for every one hour you spend recording.