German & Austrian Christmas markets usually have very overpriced, tatty stuff (we live in Austria). You can easily spot the ones reselling mass produced things from China a mile away. Unless you find one which has genuine artisans and small local brands. Some markets do make an effort to curate their sellers and those are quite fun to visit. Basically like a craft fair with Christmas vibes.
However even then, many "handmade" things are a bit meh and nothing you couldn't get as gifts back in the UK. Think candles, scented soaps, essential oils etc. The Lebkuchen hearts are cute but be wary because they are often hung as decoration, and not meant to be eaten despite expiry dates. We've ended up with mouldy ones on several occasions.
The fresh food and drink however is usually fantastic. So as a rule, all the freshly cooked things and alcoholic drinks are worth spending money on (sausages, meat, bread, fried foods, potatoes, donuts, sugared nuts etc). But don't purchase anything edible that are pre-made or pre-packaged such as gingerbread, cream rolls, chocolate covered fruit etc. Those are usually not fresh at all and left out day and night without proper refrigeration. Daytime temps of 10-15C are common in November and December, much higher than food safety recs.
Local people rarely buy gifts at Christmas markets. It's really a place to eat and drink. So don't worry too much about luggage allowance...you won't miss out on much by not getting gifts there!