I am no expert at making Belgian beers (although I have read a lot) and do not have the experience on how high gravity beers will attenuate if the sugars are added to the boil rather than to the fermenter after fermentation starts to slow. There are others on this forum that are experts on high gravity Belgian beers.
My single experience with brewing a Rochefort 8 clone that started at 1.076 and ended at 1.018 turned out great but is slightly sweeter than I would like. My process included adding the dark candi syrups (about 15% of total pounds) to the boil kettle and aerating the wort only through splashing (which works just fine for me as most of my beers start at around 1.050 and I pitch healthy starters). Based on this single experience I did not obtain the high attenuation expected from the Rochefort yeast. This could be due to many factors including not oxygenating the wort, adding the syrups to the kettle, or my mash schedule. Based on this result, I will be adding the sugars to the fermenter the next time after fermentation slows.
All that being said, my Rochefort clone turned out great even though I missed my numbers so whatever path you take you will make great beer and enjoy the journey.
Below is a snapshot of Gordon Strong's Belgian Dark Strong recipe input into my program that is set to my brewing system inputs. The syrups and sugars are added to the kettle as noted in his recipe. His recipe has a very high 1.091 OG and the sugars/syrups amount to 12.4% of fermentables by weight and he still achieves very high attenuation. Here is the link to his recipe...
https://4b88jq3r6efj3m2kjq2ft5hh-wp...merce_uploads/Belgian-Strong-Dark-Ale_rev.pdf
While the %s of fermentables are the same in my program, the quantities are different from his recipe probably because I lose more wort during transfers. I had to deviate from his mash plan as he uses a step mash that is raised by heating rather than adding boiling water. The high OG combined with my 10 gal mashtun makes doing his step mash schedule impossible for my system - you should confirm that your mashtun will handle such a high gravity beer.
What original gravity are you targeting? What is your recipe? How much sugar/syrup are you adding?
View attachment 710832
View attachment 710833