After @BrianH put me synths to use with his brilliant soaps I have noticed a marked improvement over boar in lather in slickness and protection BUT the Yaqui I have and the omega Roma feel a bit ‘functional’ and do not have the character of boar, so much so that shaving is less fun..
G5B stands out in my collection - soft tips and a scrubby backbone. Some folks suggest that it is an attempt to bring the character of horse hair into the synthetic line.
That one at Traditional Shaving certainly looks like G5B despite not using the B in the description. At 21mm, it's quite ... well, traditional. 24mm and up seems to be the thing nowadays, but then again it'll likely feel like a ... well, traditional brush in character.
Low budget I would go for an Omega S brush (their take on synthetic boar fibres, S stands for Setola which is the italian way of calling boar bristles)