I would start with confirming how much "available" payload capacity your 2018 Highlander has for supporting the "loaded" tongue weight of a given TT and WDH weight.
I would take your Highlander to a CAT scale (3 minutes and $9) and weigh it under loaded conditions (full fuel, passengers, typical camping cargo, etc.). Subtracting the CAT scale weight from your Highlander's specified GVWR will be your "available" payload capacity.
When looking at new TT's figure 15% of the TT's specified GVWR as a worst case scenario for your "loaded" tongue weight of the TT in question. An ideal loaded tongue weight should fall in the 13% to 15% range.
Also, when looking at new TT's look for the "yellow sticker" on the TT's outside wall specifying the actual "as-shipped" UVW...., ignore published UVW's (brochures, web sites, etc.) because as-shipped UVW's are always heavier.
Bob
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2016 GM 2500HD 6.0L/4.10
2018 Jay Flight 24RBS
2002 GM 2500HD 6.0L/4:10 (retired)
2005 Jayco Eagle 278FBS (retired)
1999 Jayco Eagle 246FB (retired)
Reese HP Dual Cam (Strait-Line)