Thanks for your help. First of all, congratulations on your success at gaining weight so far. It may take time for your immune system to stabilize and your body to settle in at your previous or usual body weight. In the meantime, it may well prove to be a struggle to keep yourself built up, as you have noticed. But your solution is a good one.
Liquid supplements can be easier to swallow and less filling than solid foods.
A dietician could help you calculate how much food you would need to be replaced by supplements, if you worked out a less burdensome diet plan. It may be a good idea for you to talk to a dietician. The type of protein almost surely does not matter, in terms of building yourself up. The grams of protein should be enough to replace whatever food you removed from the diet. There should be lots of calories, too.