Best underwear type for guys with smaller penises?

So I am a 22-year-old guy who currently wears loose-fitting boxer shorts in L or XL size. I have been wearing boxers since I was about 12 or 13.
However I am quite slim as I have quite a small penis (around 2 1/2 inches flaccid and around 4 3/4 inches erect).
However I find that the tip of my penis is often a bit sore as it rubs against my boxer shorts a lot (this is especially true for the ones with rough material but the ones I have with soft material are better for this). This is especially true if I go on the exercise bike it can get uncomfortable sometimes as my penis is continually rubbing against my boxers. I think the reason for this is because I have a small penis because with most guys it bulges in their underpants so that it doesn’t rub against their underpants (like you see on models who wear underwear).
I also find that if I masturbate too much my penis feels sore as well.
I have tried buying boxer-briefs but I haven’t been able to wear them as I made the mistake of only buying medium and because of my IBS, this makes underwear with tighter waists uncomfortable.
I am considering purchasing some Calvin Klein low rise boxer briefs in size XL for myself.
I’ve also thought about buying traditional underpants such as tightie-whiteys that I know a lot of guys hate and I can kind of understand why as if you’ve got a big penis they must be pretty uncomfortable to wear especially if you get an erection while wearing them. (shudders)
However, as I’ve got a small flaccid penis I’m thinking they might be more ideal for me as they would give me more support and stop the rubbing problem as my **** would be supported instead of hanging loose. However, if I get an erection my penis will go to 4.75 inches which might still be uncomfortable in briefs?
Thoughts?