My 5 week old has quite a bit of reflux which often goes up to his nasal passages when lying flat on his back in his bassinet at night. As a result, he becomes very congested through his nose and we wake up to distressing noises where he is struggling to breathe. We’ve tried everything but the only thing that seems to work well is sleeping as upright as possible. It’s not feasible to hold him all night so is it safe to elevate one end of the bassinet slightly?

Have you discussed this with your GP or Child Health Nurse?
This is important if you are concerned about difficulty in breathing.
Has reflux been diagnosed by your Doctor?

You are correct, it is not feasible to hold baby upright all night.
Have you tried holding him upright for a while after feeding, then place him in his bassinet.
Red Nose recommends that babies are placed on their back & flat for sleep….... this includes babies with reflux.

Have a look at Red Nose information on reflux which includes:
Red Nose recommend that all babies, including those with GOR, sleep on the back on a firm, clean and well-fitting mattress that is flat (not tilted or elevated) to reduce the risk of SUDI.

Regurgitation in infants is normal for up to 50% of babies less than three months of age and 70% of completely healthy infants under twelve months of age. Most of this regurgitation resolves spontaneously after 6 months, and completely by twelve months in 95% of babies.
Babies with GOR (reflux) should be placed to sleep on their back from birth on a firm, flat mattress that is not elevated.
Elevating the sleeping surface for back sleeping babies does not reduce GOR and is not recommended.
If a baby is in a tilted cot, further hazards may be introduced into the sleeping environment. When tilted, babies are more likely to slip down the cot and become completely covered by bedding, or if a pillow is used to elevate the baby, pillows become a suffocation hazard.
https://rednose.org.au/article/my-baby-has-reflux