So this sent me down an absolute rabbit hole. As a DM there’s a few ways I’d consider to stop this being entirely game-breaking:

• You could argue that the only thing strength before death shows is that you can activate strength before death between hitting 0 and getting knocked out. A wizard is no samurai. Therefore concentration spells are not allowed.
• You could argue that life steal requires life to steal, and as such you can’t life steal yourself.
• You could enforce the requirement of the figurine required for vampiric touch, then engineer a scenario to remove it at a critical moment and see if they realise.

Personally I would instead depart from RAW and point out a version of option 2, but a lenient one. Something like “you can do this but you are sapping your very essence to do it. Every time you do it, you permanently lose 10% of your HP” or “every time you do this you increase the number of death saving throws you must succeed before you die”. Or my personal favourite: “every time you do this you perturb the very laws of nature. Nature is rather fond of its laws and so decides to perturb you right back. Roll on this table to see what happens.” and make the table include the above alongside a few other things and maybe a roll on the wild magic table.

In the end I enjoy ingenuity but the role of DM gives you a lot of latitude to… handle… those who believe they found a loophole.