I was below goal weight at 7 months so I increased from 600 calories to 1000 over several weeks then over a few months up towards 1500/1600 where I am now to maintain. Initially I added nut butters, I switched from low fat yogurt to 4%. I drank 2% Fairlife milk to up calories since it added nutrition and calcium. It took a long time to get more food in volume wise. I did start adding some slider foods but purposely chose things that were not very appealing like boring graham crackers or adding granola to yogurt. I didn't want to add tempting sliders like bread, pasta, pizza or donuts etc that could cause bad habits. Now at 1 year out I do have a piece (100 calories) of dark chocolate a few times a week and 1-2 cookies or 1/4 cup of regular ice cream 1-2 times a week. I have even had a few chips but I put them on a plate, never eat out of a bag. I was so scared of going into maintenance that I would over eat and make poor choices but I feel like I have a good handle on it now.
Just remember to try a little of new food slowly to see what you tolerance is. I don't dump but get stomach discomfort from some foods.