If there are no constraints, then it is easy to get sufficient protein on a vegan diet.
On the other hand, with constraints, which in my case were that I was not willing to spend more money for a vegan-based diet than for a meat-based diet, and that I was not willing to spend more than one hour per day with cooking, cumulated for all the meals of a day, while also wanting to eat mostly, if not exclusively, food cooked by myself from raw ingredients, in order to have complete control over the composition of the food, then it becomes difficult to find a solution for adequate sources of vegetable proteins.
At least in Europe, where I live and where tofu and the like are expensive, I could not find a solution within these constraints for a long time.
You have mentioned seitan, which is just another name for gluten protein extracted from wheat floor. This has been the start of my solution, because gluten is the only vegetable protein that can be extracted at home from a cheap food, without any special equipment and without any chemicals except water.
However, while gluten a.k.a. seitan is cheap, extracting pure gluten from wheat dough requires more time and more water than I was willing to spend. The breakthrough happened when I have realized that I do not have to keep washing the dough until all the starch is removed and I obtain seitan. It is enough to wash for a few minutes until I remove about 75% of the starch.
Then I can bake the dough and I obtain a home-made bread that is highly enriched in proteins, with about a 40% to 50% protein content. Now I make every morning for breakfast such a bread from 500 g of wheat floor (much of which is dumped as starch that does not remain in the baked bread), which provides a little more than a half of my daily protein intake. The rest of the proteins come from various vegetables, including enough legumes for an adequate daily intake of lysine.
The number of supplements that a vegan needs to take depend on the cooking methods that are used. For instance I use some cooking methods that remove some undesirable substances from vegetables, but they also remove some of the necessary nutrients, so I add some supplements to compensate for that.
With other cooking methods, the supplements would not be needed, but then the substances that are not removed by cooking could be harmful.
A few other supplements are optional, but without them it is difficult to compose a daily menu with enough of everything, especially when you have a sedentary work so you must eat little to keep your weight. Such optional supplements allow much more varied daily menus.
Besides the supplements mentioned above that can be omitted, there are supplements that are needed by any vegan, because they either do not exist in plants or they exist in a too small quantity and they either cannot be produced by humans or they can be produced only in a too small quantity, so the studies made on vegans have shown that these substances are present in smaller quantities in the bodies of vegans than in those following a traditional diet.
Among this last kind of supplements are: sodium (table salt is the most ancient nutritional supplement, it is strictly needed by vegans, while with enough animal food it may be not necessary) and calcium, iodine and selenium (some plants that are rich in sulfur, like garlic or onion, may contain some selenium because they extract it from the soil together with the sulfur; however this selenium content is unpredictable because it depends on the soil that happened to be used for cultivation; whoever does not own a chemical analysis laboratory cannot count on a known selenium content, so a supplement is necessary), vitamin B12, creatine and choline and taurine, DHA and EPA omega-3 fatty acids, vitamin D3 and vitamin K2 (the humans make vitamin K2 from the vitamin K1 that is found in green leaves, like they also make vitamin A from the carotene found in plants; however while the efficiency of conversion from carotene to vitamin A is known, so e.g. 100 g of carrots per day are certain to provide enough carotene to cover the need for vitamin A, the efficiency of conversion from K1 to K2 is not known and it is not known whether it is possible to eat enough K1 to eliminate the need for an addition of K2; so until further data the prudent choice is to also take K2).
So the above list counts 12 chemical substances that are needed by all vegans, at least until new studies will determine more accurately if they can be substituted in any way. That does not mean that one needs to have in house 12 different supplements, because many of them can be found combined. For instance I use an oil with DHA+EPA and another oil with D3+K2, which I mix in small quantities with the vegetable oil added to food, while the others are either powders that I mix in small quantities with the table salt, or they are taken in a capsule per day (B12, iodine and selenium).