That's the crux of the issue. Identifying a legitimate committee purpose is a BIG stretch. That something 'might' be in them isn't a legitimate purpose. There are also certainly other ways to determine if there is a conflict of interest, or get a listing of his debts (which he provided in his financial disclosures).
What you just listed is really the 'crux of the issue'. HE provided his financial disclosures, which doesn't really carry any credulity to it since he's a pathological liar and would say or do anything to protect his illegal activities. Everyone is focused on one investigation, the Mueller investigation when in fact there are multiple congressional committees that have ongoing investigations. It's April and they're all just beginning to kick into gear. Trump's life is going to be a living hell for the next two years.
The Intelligence Committee which has the investigatory agenda on Russia as well as other intelligence-related topics.
Oversight Committee which is the House’s main watchdog for the executive branch. But for the past two years, the Republicans running it have spent little time on oversight of Trump’s appointees.
Judiciary Committee where if Trump were to be impeached, the process would start in the House Judiciary Committee
Ways and Means Committee which is seeking to obtain the past six years of Trump's tax returns
Financial Services Committee which has the ability to scrutinize broad swaths of the financial industry and agencies such as the Consumer Financial Protection Bureau and the Department of Housing and Urban Development
Foreign Affairs Committee which plans to dig into Trump’s connections abroad and whether his business interests might be influencing the administration’s policies.
Energy and Commerce Committee which has some of the broadest jurisdiction in Congress. This committee plans to examine how the Trump administration, led by a president who denies climate change even exists, is neglecting or even exacerbating the problem.
They will also spend a lot of time on the Environmental Protection Agency, which has rolled back Obama-era regulations governing coal and methane while also disbanding an air pollution review panel.
Natural Resources Committee which plans to investigate both policy and ethics matters from Ryan Zinke’s controversial tenure. On Zinke’s watch, the Interior Department proposed the largest rollback of federal land protections in US history and opened nearly all US coastal waters to offshore drilling.
Veterans’ Affairs Committee, One of the potential items on their agenda for a probe will likely be the ProPublica report in August that found that three members of Trump’s Mar-a-Lago resort — Marvel Entertainment chair Ike Perlmutter, Palm Beach doctor Bruce Moskowitz, and lawyer Marc Sherman — were essentially calling the shots at the VA, reviewing policy and personnel decisions from the get-go.
Science, Space, and Technology Committee will focus more on how federal agencies under the Trump administration are handling climate science and research matters — scrutinizing, for instance, the EPA’s ousting of science advisers and its proposals to limit the kinds of research that can be used to make regulations.