Change the linespacing locally in a package file

I am making a titlepage for my PhD thesis, in the memoir class, and I want to increase the line spacing slightly in the title.

How can I increase the line spacing? I have tried the linespread{1.5} command, but it has no effect.

Package file contents (filename UiTThesis.sty):

%%%% file UiTThesis.sty
NeedsTeXFormat{LaTeX2e}
ProvidesPackage{UiTThesis}[2020/06/19 v0.1 UiT PhD Thesis]

% Check that the memoir class is loaded
@ifclassloaded{memoir}{letendUiTThesisrelax}{letendUiTThesisendinput
    PackageError{UiTThesis}{The UiTThesis package only works with the memoir class}%
    {@ehd}}

% Load packages
RequirePackage{lipsum}
RequirePackage{graphicx}
RequirePackage{pdfpages}
RequirePackage{microtype}
RequirePackage[colorlinks=true,
                linkcolor=blue,
                citecolor=red,
                linktoc=page]{hyperref}
                
RequirePackage[backend=biber,
                maxcitenames=99,
                mincitenames=1,
                style=numeric-comp,
                maxbibnames=99,
                firstinits=false,
                hyperref=true,
                uniquelist=false,
                uniquename=false]
                {biblatex}
                
                % Make group of references that will not
                % appear in the references section
                % (citing papers in List of Papers)
                DeclareBibliographyCategory{dontbib}

% Page layout
setstocksize{240mm}{170mm}
settrimmedsize{stockheight}{stockwidth}{*}
settrims{0pt}{0pt}
setlrmarginsandblock{2cm}{3cm}{*}
setulmarginsandblock{3cm}{3cm}{*}
checkandfixthelayout


% Define a title page
newcommand*uittitlepage{
    begin{center}
    begingroup
    vspace{0.1textheight}
    %rule{textwidth}{2pt}[baselineskip]
    {hugebfseries@title} [baselineskip]
    {largeitshape Subtitle}
    %rule{textwidth}{2pt}[baselineskip]
    vfill
    {Largebfseriesscshape@author}[baselineskip]
    vfill
    {largeitshape Submitted in partial fulfillment of the requirements [0.5baselineskip] for the degree of Philosophiae Doctor}[baselineskip]
    vfill
    {largebfseries Main Supervisor}  Firstname Lastname [baselineskip]
    {largebfseries Co--Supervisor}  Firstname Lastname
    vfill
    large@date
    endgroup
    end{center}
}

endinput

MWE file

documentclass[11pt,
               twoside,
               onecolumn,
               openright,
               final]
               {memoir}
               
usepackage{UiTThesis}

author{Firstname Lastname}
title{A Very Scientific Title}

begin{document}        
    begin{titlingpage}
        uittitlepage
    end{titlingpage}
end{document}

begin{titlingpage}
centering

hugebfseries My large and long title with too small line spacing

largeitshape Subtitle

Largebfseriesscshape Author % write by hand
vfill

largeitshape Submitted in partial fulfillment of the requirements
for the degree of Philosophiae Doctor


largebfseries Main Supervisor
Firstname Lastname

largebfseries Co--Supervisor Firstname Lastname


large date % write by hand
end{titlingpage}