Recent research has focused on single antenna radio-frequency (RF) and terahertz (THz) wireless systems to mix the access link with the backhaul. In this paper, we evaluate the performance of a mixed RF-THz system employing multiple antenna-assisted access point (AP) for the RF link and single-antenna THz transmissions. We employ an equal gain combining (EGC) receiver at the AP and use the fixed-gain amplify and forward (AF) relaying protocol to interface the RF and THz links. We derive analytical expressions for probability density function (PDF) and cumulative distribution function (CDF) of the end-to-end SNR for the considered system assuming independent and non-identically distributed (i.ni.d.) $α$-$μ$ distribution to model for both RF and THz channels and pointing errors in the THz link. We analyze the system performance using the outage probability, average bit error rate (BER), and ergodic capacity involving bivariate Fox's H-function. We use the residue method to develop asymptotic analysis using Gamma functions to show the impact of the various channel and system parameters on the outage probability and average BER in the high SNR regime. We use computer simulations to depict the scaling of the performance with an increase in the number of antennas at the AP for signal reception in the access link.