ill take a look what topping has.
If you decided on the separates option then, for the amp, their A50S would be worth a look.
The frequency response is as flat as a pancake, noise is non-existent and it will drive any headphones (balanced or unbalanced) with no problem. (I use it balanced to Sennheiser 560S - said by some to be the poor man's 660S - found by me to be better than the 600).
If you can buy better than the A50S (at doing its single job), I'd like to know how.
A perfectly good DAC does not need to be expensive - checkout the original Khadas Toneboard.....a bit skeletal looking, but cases are cheapish, or you can place it out of sight.
Something of that quality and price, plus an A50S, wouldn't be much more than your £300 budget.
As you had a Zen though, you might miss the convenience of having DAC and amp in the same box 🤔.