Good work on the repair
.... I gotta get a bigger mill someday.
How about a picture of the end of the boom where it is cut?
Premacon & THS split more than a yr ago.
My Fumotec models use an ABS bushing in the two plates - the cradle that the boom fits too. You probably should consider doing something to prevent oblonging holes.
My 850 is running 4s, but the pump & motor have a belt reduction of 2,5:1. On 4s the pump is spinning approx. 4000rpm. I see your model is direct drive with no gearhead reduction. I have had no problems running the pump @4000rpm, I don't know what the limit is, but perhaps you could check the current rpm your's is rotating and go from there. From what Premacon model excavators I have seen run in videos, the hyd movements seem too quick as it with those small boom/stick configurations & small bore/short stroke cyls.
I don't think you'll need 4s, and I don't intend to use 4s either in my smaller Fumotec 490 hoe - as the bores are considerably smaller vs the 850 cyls @18 (B&B) & 21mm(S).