Has anyone gotten a 600 foot shielded cat-5 run between Mikrotiks to link at 100base?

max for cat5 runs is 300 feet.  Put a switch at the halfway point or replace it by fiber.

Its buried for 600 foot.  Running 700 foot now at 10base fine for over a year.  Need 20mbps now though.  Was thinking of shortening it too 600 foot with POE powered mikrotiks to get down too 600 and hopefully getting 100base.

Check VDSL media converters eg. Planet VC-201A.
This one is advertised as capable of symmetrical traffic for almost 100/100 Mb:

• 200 m - 90/90 Mb/s
  400 m - 90/90 Mb/s
  600 m - 70/70 Mb/s
  800 m - 55/50 Mb/s
  1000 m - 40/35 Mb/s
  1200 m - 30/25 Mb/s
  1400 m - 25/20 Mb/s
  1600 m - 20/15 Mb/s

10BASE-T use 2 pairs, split your 4 pairs cable to 2 lines then bonding (LACP) them
Just cost 4 RJ45 connector to have 20mb

Thought of that.  But I need to bond so online speedtests show near 20mbps and no out of order packets for there VOIP.  Is there a way to do that?  802.3ad prevents out of order packets but each connection flows over only one link.  balance-rr allows 20mbps connections but has out of order packets that I think would affect there VOIP system.

We plan on trenching in fiber but that will take a few months.

I was hoping shaving 100 foot off would push it over hump to do 100mbps.

VDSL media converter is transparent solution ... pure PnP