Well done OP for not giving up and finding a solution. I hope everything works out for you and your boys. Stick in there. It’s hard but hey… you are doing your best and your boys will appreciate it when they grow up enough to understand what they have put you through.
I really feel for you OP. You well and truly have the short end of the stick.
Classic Ex. Has the lols with the boys in the pub, fun Dad, whilst you get all the shit.
I have sons, more than 2 and similar ages to your OP and I know what you are talking about. It can be hard work and you are on your own without the back up of another parent. That takes loads of energy and sounds like you are exhausted. 

My older sons are at home largely due to Covid. Probably they wouldn't be here otherwise. Very similar to you OP. University, travelling and starter jobs all on hold. It's not just that easy to be out on your way in life, as much as other posters might say.
You know you really need to set down down some immovable ground rules for them and/or they move out. They are old enough for that.
I have made ground rules. They're quite basic. This isn't a student house which sometimes they forgot. Especially after 'returning'. They outnumber you OP so it's easier for them to 'forget'/take the piss.
My rules are quite basic but it works. And posters please don't tell me how yours were trained at 6 to do all these things because I thought mine were. Things evolve and change and everyone's lives are different.
Tidy up in public areas. Especially after using the kitchen. Rota for dog walks.
Their bedrooms are their own. I wince and shut the door.
I cook most nights and they like that ( no shit ). I let them know in advance if I'm not cooking.
And as it turns out OP, I now love that they are still here.
OP stick with it. Sending you energy. It's your home and not a democracy. I hope you get to see the nice and pleasurable side of your boys. I know it's under there somewhere.