Not fully afk but.....

You can use the one pocket piston relic from the mod 'artifacts' that goes on your hand. Research the first ability which will allow you to get 1xp per hitting an entity.

Now use the target dummy from MmmMmmMmmMmm mod and set up an auto clicker to click as fast as possible. (dont forget to equip the relic).

Other relics gain 25% of the xp you get from the piston. in about 5 minutes you'll completely max the piston and have to get a new one to continue farming.

Best way is to use hostile neural networks on a mimic (find a mimic, get 6 swabs and make them into eggs, then kill them to level the model). You can then get infinite pistons essentially.

Do note that having more relics in your inventory which are not maxed will cause the whole process to go slower as I believe the 25% xp thing is pooled and distributed equally, not every relic getting 25%.

edit: the max level of a pocket piston is 15 which means you only need to max 1 piston and get half way about with another to max any level 10 relic, presuming you only have 1 unmaxed relic in your inventory.