I have the Sony hybrids that DRM recommended. They are good as far as universal single silicone tips go, but may not be a great choice if you are concerned about them falling out.
I think Comply foam tips are the most comfortable universals. They use memory foam, so they are less likely to fall out of your ear and give better isolation. However, if you don't like having internal pressure in the ear canal, you might not like them. They also make the IEMs sound darker, so I would not recommend them for all IEMs.
Given your budget and concern about tips, I would advise you to consider a custom IEM. I personally find that to be the best option by far in terms of comfort (after breaking them in for 2 weeks), isolation, and sound quality. And they will not fall out. If you want a little less isolation and internal pressure, go with an acrylic shell rather than silicone. Of course, the risk you take with a custom is that it is not easy to return or resell if you don't like it. If you are already unhappy with your current universals, it may be possible to give them a custom shell. That would be less expensive.
You can definitely fall into a rabbit hole of choices so to speak when it comes to tips, since they all offer varying degrees of comfort, insertion depth, and bore widths which affect tonal characteristics and sound staging. That being said, I think the Sony hybrids offer a good balance of the aforementioned. They're definitely not well suited for every phone out there, but they're usually the tips I'll try first, before moving onto something else.
And... I think if he's perturbed by just silicon sleeves, something like customs or complys would only feel even more intrusive. That, and as you'd mentioned, customs tend to have poor resale value since most vendors won't actually reshell, leaving you at the mercy of third parties. It's for this reason that I wouldn't recommend customs to someone unless they can either audition a demo of the custom or they have a good understanding of the signature that they're seeking, cause once you commit, that's kinda it.
That aside, my own experience with customs has been pretty frustrating if I'm being honest. In both cases, I had issues with the fit that necessitated sending the customs back to the vendor in order for them to shave down the shells bit by bit til it approximated a fit I was "okay" with. And although I'd sent the UERM back twice, it's still not quite perfect, which resulted in me taking a file to the shells myself since the whole endeavor felt like the blind leading the blind. Not really UE's fault, as they were more than eager to work with me, but I was still losing time and money on my end sending the phones back and forth. The end result was that, even though it's a product that's been custom molded for my ears, I find silicon flanges FAR more comfortable in use.
Lastly, even though the UERM is one of the very best phones I've had the fortune of owning, it's still not the end all for me. Resolution, space, imaging, tonal balance, are all stellar, but I still prefer the increased resolution, improved layering and more natural sound of the FAD LAB, or the emotion/dynamics of the FIBASS like... 90% of the time, so it sits unused a lot of the time. It's kind of become like a... diagnostic tool of sorts, or a reminder of what neutrality sounds like for me. That aside, most BA setups also sound unnatural and wanting to my ears where timbre and dynamics are concerned. The FX850 still sounds more natural and capable of more finesse to my ears than the UERM for stuff like jazz and classical.
TLDR;... I don't recommend customs unless you're either prepared to lose money as your tastes change, or you're keenly aware of what you want.
Oh and Michael can't reshell the Heaven C. A large part of the reason behind the performance of the FAD BA phones has to do with the ported BAM mechanism coupled with the resonance characteristics of the metal/damping powder that they employ. Their BA tech is pretty damn interesting and one of the reasons why I enjoy prefer stuff over that of other companies
