Hi Sean,
Since the FT-818 has no tuner I’d recommend a no-tune antenna. I prefer my 3 band endfed for 20, 30, and 40m band. It has traps at 14 MHz and 10 MHZ. With a 1:49 transformer and about 5 m of lightweight RG 316 koax the SWR on all bands is better than 1.8:1 (at the band limits. Around e.g. 14.280 MHz mine shows 1.3:1.
There is one big advantage of the endfed versus the linked dipole: you can change bands very quickly (e.g. for a quick summit to summit contact) without getting up and lowering the mast to reach the links.
73
Peter