Everything you've said is right, but I'll expand a little more because ROP is fun.
ASLR, PIC (position independent code: chunks of the binary move around between executions), and RELRO (changing the order and epermissions of an ELF binaries headers: a common ROP pattern is to set up a fake stack frame and call a libc function in the ELFs Global offset table) are all mitigations against ROP, but none solve the underlying problem.
The reason ROP exists is that x86-64 use a Von Neumann architecture, which means that the stack necessarily mixes code (return addresses) and data. The only true solution is an architecture that keeps these stacks separate, such as Harvard architecture chips.
As for bypassing the aforementioned mitigations...
ASLR: Only guarantees that the base address changes. Relative offsets are the same. So to be able to call any libc function in a ROP chain, all you need is a copy of the binary (to find the offsets) and to leak any libc function address at runtime. There are a million ways for this data to be leaked, and they are often overlooked in QA. Once you have any libc address, you can use your regular offsets to calculate new addresses.
PIC: haven't yet dealt with it myself, but you can use the above technique to get addresses in any relocated chunk of code, but I think you'll need to leak two addresses to account for ASLR and PIC.
RELRO: This makes the function lookup table in the binary read only, which doesn't stop you from calling any function already called in the binary. Without RELRO, you can call anything in libc.so I think, but with RELRO you can only call functions that have been explicitly invoked. This is still super useful because the libc syscall wrappers like read() and write() are extremely powerful anyway. Full RELRO (as opposed to partial RELRO) makes the procedure linkage table read only as well, which makes things harder still.
If this is the kinda thing that interests you, I heartily recommend ropemporium.com which has a number or ROP challenge binaries of varying difficulty to solve. If you're not sure where to start, I also wrote a write-up for one of the simpler challenges [1] that is extremely detailed, and should be more than enough to get you started (even if you have me experience reversing or exploiting binaries)
Disclaimer: I'm just some dipshit that thinks this stuff is fun, if I've made a mistake in the above please let me know. I also haven't done any ROP since I wrote the linked article, so im probably forgetting stuff.
[1] https://medium.com/@iseethieves/intro-to-rop-rop-emporium-sp...