My friends and family have the exact same wealth range and I don't find I have issues navigating anywhere along the range.
For ultra wealthy friends, I don't give things much thought. I'm not into many of their spendy activities, so we tend to socialize more on my terms. If they do want to pay for me to do something with them, I just let them.
They are very, very cautious about being used for their money, but because I'm very clear that I'm really not interested in their spendy stuff, it's very transparent that I'm not particularly interested in them for their wealth. I also don't feel indebted to them when they pay for things, it also doesn't happen often, but when it does, I think nothing of reciprocity.
I also generally don't dine out with rich people because I don't like dining out. So if that's the main way they socialize, then we're not likely to hang out a whole ton because I don't like friendships based on eating at restaurants. No judgment to those who do, it's just not the kind of friendship that I prefer.
I am friends with a lot of folks who are restaurant socializers, but with me they hang out and have tea, or go out to a park, or engage in hobbies, etc.
For folks with serious financial troubles, the key is not to compare them to yourself. Your situation has no bearing on theirs. As their friend, your job is just to care compassionately about their struggles and offer whatever support makes sense.
I have very serious and often terrifying health issues, it would piss me off if my able-bodied friends struggled to hear about my health issues just because they are able-bodied.
Absolutely everyone has struggles in life, some are financial, some are health, some are professional, some are interpersonal, some are mental health, etc, etc. We all have serious shit to contend with and friends care about what each other are contending with. That's pretty much the job description of being a friend.
As for mixing friends, what would be the purpose? I don't mix friends unless I think individuals will specifically get along.
Also, lastly, I would just drop the thinking of people at "levels." No matter what wealth a person has, people are so complex that there are endless angles through which you can view similarity and difference.
For example, I'm a dual licensed professional and in each profession I have TONS in common with my colleagues, including education, career, earning potential, etc. And yet I feel I have very little in common with the vast majority of my colleagues.
I feel most at ease with folks in a rural fishing village in very remote location that I had never even visited before middle age, in a highly insular community that I wasn't raised in, where almost no one has a similar background to me education-wise or professionally.
So why do I feel more comfortable with folks I have virtually nothing "in common" with?
Because most things that we socially categorize as significant commonalities are actually very, very superficial characteristics to connect over.
I tend to feel a commonality with folks who have really healthy and supportive marriages, have a deep appreciation for nature, who live more in the present, who value connection and community, who are generous with their compassion, and who are funny as hell.
Those characteristics can come in any range of income/wealth, education, profession, location, cultural background, etc.
It just so happens that those traits are much more common in this weird-ass remote community where I bought an investment property.
So perhaps send some time reflecting on what connects you with people beyond the extremely superficial aspect of their wealth.
FTR, I'm a therapist and I spend my days talking to people about themselves and once you dip below the broad social strokes, people fall into far more interesting categories.