I really agree about cutting the slack over less important things. It doesn't mean you still can't be strict over the big, important things. IME what's a big important thing will depend on the individual child. I might get angry with my 10 year old for not doing his homework but I don't push my 4 year old to look at his school reading books if he doesn't want to.
You might want to review what's now important for your 8 year old and adjust accordingly. Is a strict 8.30 bedtime still worth a huge fight? If you get the youngest to bed earlier and the oldest goes up at 9.00 or later, will that work better? will your son feel privileged and happier? will you enjoy a bit more time alone with him in the evenings? My two sons have a bath together, then I put the youngest to bed and read him his story while the oldest goes back downstairs and watches TV for half and hour. The youngest gets a long chunk of my undivided attention, but the oldest gets a later night though no story read to him now. That keeps them both happy.
With such a big gap between my two sons - one just turned 10 and the other will be 5 in August - I know the older one can lord it over the younger one. There is friction from time to time but I fully expect this. 4 and 5 year olds are difficult for anyone to live with. Look at all the threads on here about them! All those parents at their wits end. Why should my 10 year old not feel intense irritation towards his youngest brother sometimes? He is far from an an adult himself, he is thrown together with this much younger person for hours at a time, he has to share a bedroom with him! It would be totally unrealist of me IMO to expect harmony between the two all the time and for the oldest to exhibit grown up restraint and reason all the time towards his little brother. And my younger son finds a big, loud 10 year old irritating too - and why not? A big loud 10 year old gets on anyone's nerves sometimes.
I really try and remember this. Remember that the 4 year old has different needs to the 10 year old. I am quite open with both sons that their needs are different and so are my expectations. But it's because of their age and nothing else, so that's fair. IME this keeps them happy with me and with each other and we don't have much jealousy (yet). Perhaps my sons aren't the jealous type. The oldest knows he gets more freedom and has more toys because he has had 10 years worth, but set against that, he knows he has more work to do and must be more grown up in his behaviour and set an example to his younger brother. (ie I sometimes expect him to help his brother clear up mess even if it was the youngest one alone who made the mess). The youngest knows he gets less freedom, and has fewer toys as yet, but also knows less is expected of him for the time being.
I don't know how differently you treat your two, but keeping my sons different ages in mind helps me a lot. That doesn't mean I have stopped babying and cuddling the older son. I think he needs it especially as he sees me cuddling his younger brother a lot. When he gets a bit of tlc, his mood really changes - he can't stay stroppy for long, so if you think your son wants a cuddle, don't hold back. Of course if your son talks in a baby voice simply to wind you up, ignore it at that moment and wait for a more relaxed time alone with him to baby him.
As for your oldest treating your youngest like he was his dad, can you turn this into a positive? dads are not just bossy, they are caring and protective and capable. I hate it when my 10 year old starts shouting at my 4 year old and ordering him to go to his room or tidy up his shoes. It sounds so arrogant. But I try and turn it round by getting the oldest to cook the youngest breakfast at weekends (because he likes cooking) for instance. He is still bossing around the youngest and can still be the grown up, (I hear cries of 'now EAT that sandwich I have just made you)' but at least it's in a more helpful way.
I am rambling and telling you things you know already, so I will stop. Just want to end by saying I hope you are able to get time alone with each of your sons. That IME is vital for building separate, strong relationships with them. And that in turn lessens the rivalry when they are thrown together.