Just out of curiosity, what class has access to Healing Word but not other healing spells?
What is the class(es) of your leader character(s). If it's anything but Warlord (well or Shaman or Ardent, obviously) you should be fine - as far as healing goes.
If you're sick of 4e, I'd say just start a new campaign with 5e. If you're enjoying the campaign as it stands now, I'd say finish it out in 4e. That's what my group did when 4e came out: we finished up or 3.5 campaigns before trying out the new system.
Converting a successful old campaign is a good way to make a new system look bad. Give 5e a shot all on its own, its way.
You'll want to play defensively.
Use and abuse the Dodge action. Use stealth, invisibility, and protective spells (abjurations). Get everyone a robust AC. Opt for ranged attacks where possible. Fight on your terms, not on theirs.
Healing is only one way to get "extra hits" - defensive tactics and clever strategy can also give this to you. It's just less reliable, and it makes certain approaches (like "kick in the door and deal with the consequences later!") less viable...or, rather, more likely to result in deaths.
Yeah, this is how we played it in 4e ... but all of that defensibility came from the runepriest. Gave out bonuses to defenses regularly either directly to PCs or in a sustainable area, provided always-on resists to every ally in aura 1, etc. Or if there weren't many combatants would give penalties to foe attacks. Also gave out buffs to allies to finish combats faster. Common encounter powers would do things like give a penalty to attacks equal to Wis (+6) + number of adjacent allies to a foe as an effect (plus do damage on hit), or close burst 5 give all allies +5 to all defenses and all enemies hit -5 to all defenses until end of turn (that was from a paragon path). Even at-wills can do things like have out -2 to hit foe defenses and +6 (Wis) AC to the next ally to hit it.
In other words, taking care of everything you just said, but without the other characters having to do anything, is what the runepriest is trying to emulate with a 5e character.
The party also had a paladin, but took one of the other options besides lay-on-hands. Others were an elven ranger (archer), a essentials thief and a vestige pact warlock multiclassed into barbarian. Not a lot of healing elsewhere.
In 4e was Runepriest. And the cornerstone of how that character has been played (1-14 in 4e) that is trying to be preserved is a melee combatant with heavy buff/debuffs with some healing. More a force multiplier than a healer.
Concentration makes buff/debuff and melee a bit problematic, and also takes away having out multiple buffs/debuffs for the most part. The player is looking at valor bard (most likely) or paladin for class-feature buffs, spell buffs with concentration, and melee ability. Doesn't want to spend each combat as a healbot just healing, so most heals will be bonus actions with only the occasional Action used for healing.
Both starting fresh and holding out have been proposed. Most players are absolutely against ending the campaign earlier. DM says 3+ years left (more likely 4-5, we're mid way through the second act) and two players can't deal with 4e for that much longer. We play a weeknight every other week and ever encounter have been taking up a session to a session and a half.
Thanks for your feedback. It's a bit of an catch-22 in that there is no option except continue the campaign is acceptable to the group, but the player of the only leader doesn't want to be put into a sideline role where all they do is cast heals since that's just about the opposite of how they are currently being played. Initiative allowing, the 4e shifter serene blade runepriest is usually the first to engage in melee and has a very aggressive personality in combat and out. Rather tough between serene blade temp HP each round, good defenses, and longtooth shifting for regen and a resist (latter from a feat).

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.