Go for the IVSM, maybe a home infusion nurse so you don't have to go out. When the oral doesn't help and you worsen, that means the relapse has gone acute. You need a heavy dose to get you back to your baseline.

I do 5 days 1000 mgs no taper. Or 1- 750- then 3 days 1000 mg and another 750 as the taper. It gets me walking again when I can't. I have even had 10 days of IVSM, when the relapse goes from bad to worse. JMO

Good luck and feel better.