Advice Needed

TLDR - Spend an additional $30k now to upgrade the sewer system on-site while the contractor is already mobilized or go with a sewer injection pump which is likely a viable solution the will last for years with proper maintenance.  What would you recommend?

Background:

I have a property with a duplex (3bd/1ba & 2bd/1ba), detached house (4bd/1ba) and detached garage on it. I recently pulled permits to convert the duplex's attached garages into one 2bd/2ba ADU, convert the detached garage into a small 1bd/1ba ADU, and add 1 bath and laundry closet to the 4bd/1ba house. I'll be going from a 3-unit to a 5-unit property.

As the project is getting ready to break ground I need to decide on the level of sewer improvements I should do.  The existing system is pretty old cast iron except some ABS pipe towards the front of the property before going into the street. The detached garage is in the very back, at the lowest spot of the property.  It would be a challenge to get the 1/4" per foot fall necessary to tie into the existing main line on the property, although not completely ruled out.  

Since it's low in the back the contractor bid the project to install an injector pump for the 1bd/1ba detached garage conversion to tie into the adjacent house's system.  This will eliminate the need to try to tie in at some unknown point to the main on-site.  His plumber came out and camera'ed the line and suggested the main line on the property may have issues.  He said he saw some 'bellies'.  As such my contractor gave me an estimate to install new ABS main line (~225 LF) for the property and connect all 5 units (3 existing and 2 ADUs) to it for approximately $30k.  He would tie in before the property line to avoid right of way permits and inspection.  He also suggested I bring out someone else to camera the system again to get a second neutral opinion.

I hired Cable, Pipe, & Leak Detection and the technician that came out today was very impressed with the on-site system.  He said he was able to run the full video cable 130 ft from a couple different cleanouts and didn't see any problems.  There was some minor root intrusion at the very front of the property and this tracks with my service plumber who last came out ~2 years ago and cleared some roots when the back house was experiencing back ups.  He said the system looked very good but being old cast iron he couldn't say if and when it'll fail.  

We did have a blowout at the end of the line where it daylights around 10 years ago.  If I remember correctly it was an issue again at the front of the property (probably roots) that needed to be cleared.  There was crap everywhere in the back.  It hasn't happened since but I usually have to have my plumber out every couple years when the back house starts to report issues with their sinks and tub not draining well.

At this point I'm trying to decide if I should go ahead and redo the whole system while we're out there and consider it a capital improvement benefit that will last for as long as I'll own the property. I've had it 20 years and I plan on keeping it. I think I'm still a little traumatized from the blowout we had a decade ago. It'll be nice to never have any problems with a new system. At the same time it's currently functioning with no sign of substantial compromise to the cast iron pipe beside the age. I had it camera'ed more than once (I did it this summer as well when I was doing due diligence that's how I knew the existing depths will make it hard for the detached garage ADU to tie in to the main line). Both times I was told the line looks great.

One of the main drivers of this dilemma is the low 1bd/1ba garage conversion and $30k seems like a large cost for one element of a 350 sq ft conversion. At the same time it benefits the whole property and not just the ADU. If and once it's done I shouldn't have to worry about it ever again. If I opt not to do it and just go with the pump there will be maintenance requirements for it and I'll have to rely on the tenants not doing anything that will compromise it. Also, if in the future I ever do need to redo the main line it'll be more expensive (L&M increases), I will have already likely spent thousand installing and maintaining the pump, and I'll have more tenants impacted by a sewer construction project (currently a 3 unit but will be a 5 unit at that time.)

Help.  What would you do?