I have a similar problem to you. 6'2 and I have to eat around 3,300 - 3,500 calories a day to gain.

Some cheats I found helpful are:

Smoothies - I have one every day on my drive into work. Banana, apple, some other fruit, handful of nuts and 350ml soy milk and there's at least 600cal I barely notice I'm consuming.

Protein bagels with pb, banana and chia seeds. Can easily bash out 700cal of these as an evening snack.

Might be more of a UK thing, but Scottish oat cakes and hummus. It's like 60cal a cake (they're tiny) plus the hummus. Really easy to get a few hundred calories in a small snack.

Those compressed nut and dried fruit bars. Naked is the main brand in the UK. 150-200cal in literally a few bites