Roman Legions seem too weak

[Sakkura]

Well there was an agrarian problem but it seems to be more on the order of people not wanting to go on service lest a senator grab their land while they were away, rather than an actual lack of farmers.

Yeah. Part of the social unrest from the late second century BC and on was even caused by such problems, so a reform of recruitment and/or the agrarian system was called for. The Gracchus-brothers didn't succeed with their agrarian reforms, so instead it ended being Marius and his recruitment reform that went ahead.

[Zaknafien]

not really. Marius wasnt the first to recruit from the proletariat, and even after him standard class armies were still recruited. "Marian reform" is just a handy name placed on the epoch by some people. It was really the Social War that normalized the recruitement of capite censi soldiers.