In your circumstances, I'm not sure there's any major advantage in waiting.
Significant advances in battery tech are more than two years away. It's more likely to be incremental until then. The big shifts will be towards the end of this decade. So don't hold out for major changes over the next couple of years (except possibly at the very high end, premium and large Mercedes and Porsches etc).
If you're specifically looking at a Zoe, it doesn't make a huge amount of difference really given that you're not buying new.
Renault won't be directly replacing the Zoe — the new Renault 5 will be its de facto replacement, but that won't be out until 2024 (or possibly early 2025). Used Zoe values will be hit when the new 5 comes out but not hugely, and you could conceivably be waiting up to three years anyway for the new model to launch.
The used car market is punchy right now generally but may cool a bit over the summer and into the autumn. EV values can go a bit limp when the weather starts to turn cold because a lot of people have rather blunt ideas about them being less efficient in the winter. (This is technically true but it varies model to model, and on driving style.)
My advice would be to keep an eye on used/nearly new Zoes now, work out what spec you want and equipment, and how the market behaves over them, and be ready to go when you find the one you want. They're comparatively plentiful for EVs, so you can afford to be choosy.