options for replacement banister/spindles

We have white painted Newel's bannisters and spindles - put in by the builder and not fitted at all well.

Am considering replacing but when i look at Burridges web sie the oak variants are only available for traditional fit (mortice & tenon etc).

Now I can handle basic woodwork bt not sure my sills are p to doing a reasonalbe job on this. Does anyone else do oak DIY fit?

Or am I tied to Burridges hemlock or employing a joiner?