To me, I don't care if Phil actually "needed" the money for taxes and didn't instead use it to buy a ring. Cheap ring, expensive ring, who knows? I think saying you have to visit your parents because they're dying when they're not is the lowest of the low. Yes, I know he used various weasel words to not literally say "dying", more like "they're in poor health" and "might not see them again"---but he said it over and over for MONTHS. Who says that about their own parents as an excuse to get married secretly? If your brain works normally, wouldn't you just say you're going to visit your parents because they're your fucking parents and you haven't seen them in years? But there's no pitybux or sympathy donations in that line, and that's where Phil is twisted.
Dave has this weird inherent need to justify everything he does; it even comes through when he's figuring out what games to play, what he wants to do or not do in a game or stream, and even when he brings up real life shit. Could be the pity line, or it could just be him coming up with whatever excuse he can to have a reason to do it.
If anyone wanted to dick around while they've nothing better to do, you could try to pair up when this season's tax begging arc began (does it just overlap with last season's? is this just a single, giant e-begging session spanning years?) and when the CT trip was announced. You have to assume here, but I want to bet that the announcement of the CT trip was made shortly after they were engaged (would line up 'more' if it was post-first begtrospective event since it's known the Space Needle trip happened before then and I it could be somewhat safe to assume he proposed to her there). You could be extra special and trying to find when he started the meme of, "The life Khet and I have built here," since that seems like something he might switch to after engagement, but that's nothing short of pure speculation.
I think these are important because, oddly, even with Dave's 'tism-racked schedule, he might simply have not thought it through and planned it out. I'm pretty sure he started the tax-beggery well before the trip/not-really-sick-parents announcement. With the oddity of him announcing his engagement to Reeanna a day before his marriage to Khet, you want to assume that he planned it that way in some pathetic game of spite, and he realized far too late that it was lining up exactly when he'd need to be working hardest to cover his supposed tax deficit. So it then morphs into, "Muh parents aren't doing too well, they can't even get on a plane!", and that then devolves into, "They're getting up there in age, and while they're not on their deathbeds, it's better to do it sooner rather than later," because he actually has some sort of guilt of lying about his parents nearly being dead when they're not.
I'm not a fan of this kind of speculation because you're really assuming too much and just grasping at straws, but I wouldn't be surprised if the whole ever-changing situation his parents are in is what it is purely because he didn't think far enough ahead about how the dates lined up and what lies he was currently telling. Compound that with him needing to justify everything he does in life, incapable of giving a simple 'Because I want to' as an answer, and I think you get this situation. It'd be funny if he planned the marriage/trip date solely around his failed engagement with Panda, because there was nothing stopping him from doing it in a month or two. I don't think he's stupid enough to have done it to try and gain some benefit for this year's taxes, especially if his story about having an accountant, I mean CPA, I mean a tax, uh, guy, is true, and that it's not just H&R Block. He'd be privy to the knowledge that it wouldn't help him this year.
I dunno, there might be something there, probably not. Whatever.