Regarding THE BUBBLE:
There's a bit more going on here than the TL alludes to. First two lines are pretty obviously "Sayonara, Sumireko-san", but that third is
very tricky to parse. I've never been able to sort it out to my personal satisfaction myself, so I'm just gonna crowdsource this in the comments and see what happens:
From what we CAN see, figuring out that first kanji seems to be the key to unlocking the whole thing. It'd be a better fit if I had the actual font that the manga uses but this one seems fairly close enough. In any event, you have to work it out by recognizing that upper-right slice, which is tricky because you also need to be able to recognize the implied width of the kanji as well. If you've been reading Japanese all your life, you can probably do this instinctively (which makes it double hard for translators because Nujima is trying to be ambiguous to NATIVES here). If not, then you start resorting to overcomplicated pixel analysis like this. Anyway, the upshot is that 野 and 疑 seem like they could fit where 柔 or 通 seem off. Or maybe it's not even any of those! There are a lot of kanji.
There's also the second and third characters to consider. For instance, that 3rd one probably isn't 'ma' because it doesn't align with that little visible part on the bottom very well.
So what is it? Every time I've thought about it and decided on something, a few weeks later I look at it again with fresh eyes and then decide I was totally wrong before, so hell if I know. (Astute readers of the originals will notice that 野 "fits" and also is the 'no' in 'Adashino', and we alllll know how mangakas like their character names to be laden with MEANINGS, but I never managed to fill-in-the-blank with it here in a way I felt good about.)
TL;DR: there's
something else to Ren's "last words" to Sumireko, but personally as a translator at best I'd be throwing out a slightly-educated-mostly-wildass guess and TL noting it
edit: but if you put a gun to my head and say I DON'T CARE TELL ME WHAT YOU THINK IT IS then I'm going with 野暮でした, literally "jackassy/dickish/uncouth/etc [past tense]", or in context probably more translated as "Sayonara, Sumireko-san, sorry for being a jackass". It's not a literal apology, but to say goodbye forever and also acknowledge he acted like a jackass in the past is basically saying sorry without formally saying sorry.
edit 2: Might I direct you to
@manshiro a few posts ahead who is much better at staying out of the radical weeds than I am:
I'd like to think he just decided to go all in on the romance and went with the classic "好きでした" (I loved you) drowned out by fireworks.
This is 100% correct. It's "Sayonara, Sumreiko-san. I loved you."