You think that when Congress votes to send aid to an American ally, the president gets to make said ally jump through hoops for the president's personal purposes before that aid is unlocked? It's no different in essence from, say, a US customs officer who refuses to let an otherwise qualified tourist enter the country unless the tourist agrees to pay him money, or to wash his car, or to do his dirty work digging for dirt on the officer's enemies. Just because he's the president doesn't mean he gets to trade government resources for personal favours, any more than that customs officer. And if you believe for a minute that investigations into Crowdstrike or into Hunter Biden were legitimate US government interests, instead of personal favours for Trump and his re-election campaign, I've got a few bridges to sell you.

I've seen some Republicans arguing that this isn't proven, that somehow there could have been another perfectly legitimate reason for withholding the aid so long, that it's a perfect coincidence that suddenly, as the story became public and the difficult questions started to come, these mysterious other reasons no longer applied and the aid could quickly be sent. Which is, to put it mildly, a pretty implausible story - though it has the benefit of acknowledging that if indeed it's proven that the quid pro quo happened like Sondland says it did, that is a big deal.

I've also seen some argue that although what Trump did was certainly bad, it was not quite so bad as to make him deserve impeachment. My understanding is that the Constitution is rather vague on where precisely to draw the line, so that conclusion might be a legitimate judgement call. But then they don't get to describe the impeachment as a 'witch hunt' or 'abuse of power' or whatever, it's merely the Democrats being understandably less inclined to give the president the benefit of the doubt.

And that blocking of testimony, and refusal to hand over any related documents, would be where the 'obstruction of Congress' comes in. Based on your paragraph about that, it seems you haven't read it and overlooked that it refers specifically to him stonewalling Congress in its investigation into the Ukraine affair. Of course you will argue that it's a matter for the courts to decide to which extent the president can refuse Congress' demands for documents and testimony from government officials. Even though so far all court cases on the matter have put Trump in the wrong on this, he kept appealing so the Supreme Court will have the final say. Hence you can argue that it makes no sense to have an article of impeachment about it until the SC confirms that indeed the president must comply - and if he then does comply, it's no longer a reason to impeach. I think that's a plausible line of argument, the second article seems pretty questionable to me.

But apparently the Republicans had a different view twenty years ago on whether such obstruction should be grounds for impeachment, considering their 4th article in the Clinton impeachment:

"In responding in such a manner, the president exhibited contempt for the constitutional prerogative of Congress to conduct an impeachment inquiry," the GOP report says. The "answers are a continuation of a pattern of deceit and obstruction of duly authorized investigations."