I gained 25kg on a vegan diet through a span of 5 years. You just have to eat a bit more than comfortable but protein digestibility is a non-issue. Stuff that I ate - tofu, seitan, tempeh are very low processed soy/wheat products and can very quickly give you more than 200g of protein a day.

For me, the biggest problem was meeting caloric needs. It's very hard to get to 3000kcal+ eating non-processed plants.

PDCAAS, DIAAS or similar "bioavailability" measurements are standardized on pigs bowels and are targeted towards malnutritioned individuals that border on starvation or suffer from kwashiorkor.

As for weightlifting, you can do rounded back (inefficient form) deadlifts smartly (not overdoing the load) and get strong enough.