Our Bub is 7 weeks old and gets very congested and gets reflux when we lay him flat on his back. If there are no blankets being used in the bassinet, is it safe to use a crib wedge under the mattress to elevate him? I’ve read a slight elevation has helped others with this issue, but have also read caution due to the infant slipping down in the crib and under blankets etc. If the blanket hazard is removed, what are (if any) the other risks to using a crib wedge?

Red Nose does not recommend elevating a baby’s sleep surface.
As you have noted, one of the reasons is the likelihood of baby slipping under any blankets in the sleep environment, risking the covering of head & face.
Even if there are no blankets, when baby slips down any incline, they are no longer elevated.
Also, a baby’s swallowing reflexes work more effectively when baby is on their back & flat.

Red Nose provides this information.

Gastro-Oesphageal Reflux (GOR) is the effortless regurgitation or spitting up of gastric (stomach) contents into the oesophagus (food pipe) with or without effortless regurgitation and vomiting
GOR regurgitation or ‘spitting up’ is common in babies. Regurgitation occurs in about 50% of babies less than 3 months of age and resolves spontaneously, without intervention, by 12 months in all but 5% of babies.
It is usually mild and self-resolving. GOR is a normal physiological process occurring several times per day in healthy babies both term and preterm and is normally cleared by swallowing
Elevating the sleeping surface for back sleeping babies does not reduce GOR and is not recommended.
If a baby is in an elevated cot, further hazards may be introduced into the sleeping environment.
When elevated, 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
Sleeping baby on the back provides airway protection. The back sleeping position is safer for babies with GOR as babies can protect their airways when placed on the back compared with babies placed to sleep on the tummy or side
https://rednose.org.au/article/sleeping-position-for-babies-with-gastro-oesophageal-reflux-gor

If you are concerned that your baby may have Gastro-oesophageal Reflux Disease (GORD), it is important to be assessed by your Doctor or appropriate Health Professional.