If you can return it, I would do that. What you're experiencing could be seen as a blessing (many are experiencing it and similar results from untested updates, but no longer can return our tvs. I would take the opportunity to get out if I were you.).
Your next option would be to open a warranty claim with the manufacturer. The manufacturer will probably say it's Roku's problem (which is true in many ways). But, you bought Onn's tv, not Roku's. It was Onn who entered into a licensing agreement with Roku, and offered a warranty to you. Maybe Roku will feel the heat from their partners. Lord knows they don't care what customers think. (If you feel your rights to warranty coverage are being denied, file a complaint with the FTC & your state AG.).
Your next option would be to cut your losses by doing a home>settings>system>advanced>factory reset, but tell it you don't have internet access. Get back to the older version of software the tv came with. Never connect it to the internet. Never let it be subjected to Roku's untested updates.
This would let you use your tv as a "dumb tv." You could watch antenna tv. If you want to stream, spend $30'ish (USD) on an external streaming device connected to HDMI. "Unbundle" the tv. "Cut the cord" with Roku. Use your TV as a tv, and keep your options open with plug-n-play isolation of functionality. If you don't like the idea of using two remotes, look at the URC-7935 (programmable & learning. It should have codes for Roku & any streaming device you buy. But, also the ability to learn from an existing remote.).
If you look back through this forum (go back pages), you'll see there's not a lot of reason to be hopeful. Roku's been pushing this buggy update for months. They know the problems it causes. They don't care. They kept pushing it, and still are. (There are problems which have existed for YEARS. Never acknowledged. Affected customers left in silence hoping for nothing.). It's possible something will change at Roku. but, I don't see any reason (in the slightest) to expect that.
There's no bug tracking/sorting/prioritizing. You'll be asked to provide the tv details which will then be casually "passed along" to someone who never acts on anything. There's no care involved. Your bug report is more like advice that they'll take in stride. It's not really a problem, or anything.
"People are often amazed at how much we’ve done with the number of engineers we’ve got." (Roku CEO Anthony Wood, Austin Statesman, Oct 4, 2019). "Amazed" is one way of putting it.
