If you don't mind risking less than half the cost of the SP-445, you might give the Yankee Agitank a try. Biggest downside is that you need 1.6+ liters to cover 4x5 sheets -- but that same volume of developer will process anywhere from one to twelve 4x5 sheets. Master the agitation technique (tilt one way, back to level, then the other way and back to level is one agitation cycle) and you'll get even development; it fills and empties almost as fast as a Paterson Super System 4, or your could use three the way you did your Combiplan tanks.
Use the Agitank with replenished developers, and it's economical; use it at full capacity, it's economical -- don't use it for a single sheet with one-shot developer unless you really don't care about process costs.
I have one and have no plans to replace it; I replenish either Xtol/EcoPro or D-23 (as well as Flexicolor C-41), so it costs me no more per sheet to develop a single sheet in it than to use it at capacity.
I see only two significant downsides: it's easy to misload, putting two sheets in a single slot at one or both edges, and it seems to be made mostly of phenolic -- generic for Bakelite -- which is brittle; if you drop it, or drop something on it, it's likely to break.