SLVO & GLDI aren’t bonds, but covered-call ETFs that offer fat yields. The monthly div isn’t fixed, cumulative, or guaranteed. But then, neither are the coupon rates on many preferreds. If you’re willing to accept the risks of non-cumulative, floating-rate, never-maturing preferreds, then you should be (at least) looking at covered call ETFs, of which there are a dozen or so that cover the major asset classes/indexes.
Right now, SLVO & GLDI are trading lower than they’ve been in a year. Due your due diligence and --if you buy-- size your opening position small. (IMHO, 'natch.)