Did you back flush the heater core?
Or at least check the flow thru the core with garden hose?
On my '94, Heater core cost under $30, 2 hose clamps, 4 screws and it is out, takes about 30min to change, if you had to do it again it would take under 15min. lol.
My original lasted almost 20 years so no complaints.
Do this run engine up to temp and feel both heater hoses, both should be hot, now turn on heat in truck and set fan to High.
Wait a few minutes and feel both hoses again, if one is getting much colder then there is not enough coolant/water flowing thru core to keep it warm, fan is blowing all the heat away from what little flow there is, tubes in core are restricted or some are blocked.