I think unless someone is an experienced iPhone repair professional...or a DIY person who has done this specific repair...I think an exact answer is hard to say.
For us amateur DIY folks...the best we can do is replace suspected bad parts for known good parts. In this case the best you can do is purchase the parts...replace them...then see the results.
Sounds like replacing the motherboard is the first thing try (especially since the guy at the local shop said he thought the issue was due to a damaged motherboard).
If you replaced the motherboard & the dim display issue remained...then it would be time to try replacing the display (if the display is a possible source of the issue).
For both parts (motherboard and/or display)...would be best to purchase them from a place that accepts returns. This way if they don't correct the issue...you can return them.
Another solution to consider (if you got to have an iPhone 6)...is purchase a used iPhone 6 via eBay or somewhere else. iPhone 6's are pretty old...and a good condition used one may not be that expensive. Worst case you need to replace the battery.
HTH,
Nick